Yes, Ted did have an interest/fixation on snow rabbits, and the possible Zodiac Christmas card of 1990 features a snow rabbit. Ted was also using in winter 1990 the same stamp the Zodiac was using in winter 1990.

There is a pattern of unconfirmed but possible/probable Zodiac letters being mailed from places Ted was at and with the exact same stamps that Ted was using at the time, as the Unabomber and/or in his personal life. This happens in 1986, 1987, fall 1990 and winter 1990.
There is one unconfirmed Z letter that has never been talked about much. I first heard about it through Dennis Kaufman and that is a bad start right there. But apparently the expert at the Cal Justice Dept confirmed it as probably authentic. That expert is said to be Terrence Pascoe, also known as Terry Pascoe. I have since talked to two different credible people who say Pascoe told them it was a confirmed Zodiac letter. It does look like Z writing, to my unexpert eye. And it lacks a zip code, something consistent with all the confirmed Z envelopes up to that point. Yet it never got much publicity.
The handwriting IMO does appear to match. The majority of posters here seem to think it is probably fake, if they even know about it. And perhaps it is fake, but at least four things hold me back from being 100% sure it was a fake:
1. The Cal Justice expert Terry Pascoe apparently thought it was probably real and is said to have confirmed it as a real Zodiac letter. And it looks like Z writing.
2. It somewhat fits a rough pattern of letters appearing about every four years: Three confirmed Z letters in 1974, three (probably mostly or all fake) Z letters in 1978, several unconfirmed Z letters mentioning the Atlanta child murders in 1981/1982, this 1986 Z letter and the 1990 Eureka Z Xmas card and celebrity cipher card.
3. There is compelling evidence Zodiac suspect Ted Kaczynski was in Northern California at the times of the 1990 possible Z cards sent from there, and it is documented he was in Sacramento around the time of this letter, as he killed a man with a bomb there in December 1985. In this 1986 letter Zodiac mentions having just killed people in Sacramento by a freeway, and Ted Kaczynski had just killed a man via a bomb in Sacramento by a freeway on December 11, 1985. Ted used the same Christmas tree stamp as used on the 1990 envelope, and he also used the same flag stamp as used on this 1986 possible Z envelope in two letters in 1985.
4. Ted Kaczynski did have and use this
exact same stamp as was on this possible Zodiac letter from 1986, the flag in front of the Capitol Building. From the Turchie affidavit used to get the search warrant:
92. United States Postal Stamps - All of the ten mailed devices utilized United States postage stamps, as opposed to having a postage metering strip. Nine different types of stamps, in varying combinations, appeared on the ten different devices. Four different stamps depicting the United States Flag were used. The first was a 29 cent, U.S. Flag with the Olympic Rings, which was first issued in April 1991. This stamp appeared on devices 13, 14 and the letter to the New York Times, dated June 24, 1993.
The second stamp depicts the U.S. Flag in front of the United States Capitol. This 22 cent stamp was first issued in March 1985, and appeared on devices 9, 10, and 15. Ted Kaczynski
used this same flag over the Capitol stamp on mail bombs in June and November of 1985:

That the Zodiac used on this letter of March or May of 1986:

This is the content of the letter.

So what do we have?
Ted Kaczynski purchased the "Flag - Capitol" stamp and used it on two mail bombs in June and November of 1985.
If Cal DOJ expert Terry Pascoe is correct that this is authentic, then the Zodiac used a "Flag - Capitol" stamp in a letter from May of 1986.
If Cal DOJ expert Terry Pascoe is correct that this is authentic, then the Zodiac bragged about recently "collecting" slaves "up by" Sacramento "near a freeway" in the letter with the "Flag-Capitol" stamp.
On December 11, 1985 Ted Kaczynski killed Hugh Scrutton in Sacramento, with a pipe bomb placed behind a computer store, by the Capitol City Freeway.
Hotel records and statements indicated Ted Kaczynski was out of Montana for May 1986,
leaving in April 1986 and back at the end of May 1986, mentioned to his brother he would be looking for "work" in a "warm weather state" like Utah or California and this 1986 possible Z letter was postmarked May 6, 1986.
In October 1987 there is a possible Zodiac letter, apparently mailed with a wider version of the "Flag - Capitol" stamp, part of the same series as used by Kaczynski in 1985 and the Zodiac in 1986.
What about the next possible Zodiac letters in 1990?A "celebrity cipher" card possibly sent by the Zodiac is mailed in September 1990 from Oakland, CA at a time when evidence places Ted Kaczynski in that same area and even that same city.
Ted Kaczynski purchased and used the "Christmas Tree - Greetings" stamp on a letter to a neighbor.
If authentic, the Zodiac used a "Christmas Tree - Greetings" stamp on a 1990 Xmas card to the SF Chronicle.
If authentic, the Zodiac was in Northern California, specifically Eureka, in 1990 to mail the SF Chronicle letter.
Compelling evidence, including witnesses, pictures and physical items, places Ted Kaczynski in Northern California, specifically area around Eureka, in 1990.
Suspected Zodiac Xmas Card With Tree Stamp And Kaczynski Xmas Card With Tree Stamp:
Excellent analysis by Doug Oswell:
A careful scrutiny of the composite offers some interesting revelations. Both productions were mailed in early December of 1990. The handwriting on both missives is eerily similar, even to the point of the particular character formations, the relative x-heights of the lower-case characters, the numerals, and the qualities of base alignment and extra spacing between the lines of the addresses. A test comparison of the character formation and relative x-heights can be made by culling individual letters from the Kaczynski card and comparing them with a particular word or words from the suspected correspondence:
The stamps are identical, and in the inset Kaczynski is quoted as lamenting the increased price of a Christmas card: "A buck fifteen for a Christmas card!" he complains, "This is getting ridiculous." The price of the Eureka card, as indicated on the back near the UPC code, is exactly $1.15.AK - To me, the totality of the evidence suggests that Ted Kaczynski probably did mail the May 1986 and Xmas 1990 probable Zodiac letters, and perhaps the October 1987 and September 1990 possible Zodiac letters. Ted Kaczynski owned and used BOTH of the stamps the Zodiac used in the 1986 and 1990 letters AND he was at both locations the 1986 and 1990 letters were mailed from AND the handwriting matches AND he complained about buying a $1.15 greeting card just like the Z 1990 letter AND he had actually killed a man in Sacramento just before the 1986 Zodiac letter in which Z boasts of having killed in Sacramento. Too many matches to just write off each one as a coincidence.
There are what, maybe 45 - 75 stamps one can use at any given time? And Ted Kaczynski just happened to own and use the same Capitol/Flag stamp used by the Zodiac in 1986 AND the same Xmas Tree stamp used by the Zodiac in 1990 AND was in both locations the Zodiac letters were mailed from?