4wd fishtailing
 

so driving in the partially-melted snow today, I was in 4wd on a pretty slushy road. The truck started fishtailing at about 25-30mph, and I was unable to bring it out of the fishtail until I slowed waaay down and disengaged 4wd. It also does this on dirt roads. I kept control, but it was SCARY. Anyone have any suggestions on why this happened or what I can do to fix it?

My rear and front tires don't match, could that be it? Since the t-case causes all 4 to lock together? If it's worth anything, my front tires have less tread on them than my rears.

How much of a difference in diameter is there? Are they at least the same size?

Yes, they're the same size. But the rears were brand-new in June. The fronts came off of an Exploder (worked at a tire store, needed new fronts, they didn't want them for used so I took them) and don't have a hell of a lot of tread on. When my tax refund comes in I'm thinking of either buying my friend's set of 4 used tires off his Cherokee or buying 2 new tires and rotating the rears to the front and one of the old fronts to the spare.

the other thing I was thinking about is that the rear gear ratio is different from stock (stock on my truck was 3.7ish according to the RPO sheet, it's now 4.11, PO did that), and the PO may have neglected to change the front axle ratio. But it works fine in normal offroading, doesn't bind up or anything. I'm also not offroading at 30-35mph. Normally.

Could be that the rear has a locker which would cause the fishtailing. You would know in a heart beat if the front and rear gears weren't matched.

how would I tell if it's a locker? Sorry, I'm actually kinda new to 4wd drivetrains. Also, why wouldn't it lock in 2hi?

It'll lock in 2wd, but if you have two different gear ratios on the same truck, i'd imagine it would cause some issues like maybe fishtailing

even if I only engage the 4wd on loose surfaces? Or pretty slick surfaces like icy roads? (barely matters since I live in Albuquerque and people close school with half an inch of snow on the ground-- I'm usually in 2wd unless I'm wheelin', and sometimes even then not until problems present themselves.)

Edit: What I remember from when I replaced the diff seals is that I do not, in fact, have a locker. There're spider gears in there.