Hello! First-time poster here.
Today my school got course selection sheets for our classes next year. I’ve been thinking of joining AVID, but I missed out on two years of it during high school.
I was wondering if I would still be appealing towards colleges for only taking it junior and senior year of high school, in alternative to four years.
Thanks! 
I really don’t see a specific appeal; I know that it can help a lot with prepping for college itself and aiding you in your other classes. If you want to take it, I don’t see any negative, but it’s not like colleges will treat you specially or anything because of AVID.
AVID is important if you’re a competitive applicant for selective colleges, since it flags you as first gen/of special interest in terms of demographics they’re trying to attract.
If you’re interested in college, 2 years of AVID will help in many ways, if only through the support through a complicated process, and it maymake you eligible for special programs&scholarships.
In short: no downside.
Be aware that being avid is not as easy as you might think. If it were simple, then why won’t everyone be avid and get solid grades. You should have strong dedication.