4wd ??

I have a Wrangler '88, 4 cyl. If I'm driving with 4wd low on a flat surface it pulls on all 4 wheels, but when I'm trying to go up a hill it only pulls on 2 wheels; sometimes the fronts and sometimes the rears, the other two doesn't move at all. Why is that?? How can I fix this prob?

It's really annoying since I have problems climbing even a small hill...

Transfer case problems perhaps. Should always stay in 4wd if its hi or lo. But on the other hand, it might slip. cuz when your stuck it will only spin 2 tires. I remember this when I first got my jeep, in winter time. Put it in 4wd thru a few snow banks. Got stuck, and only 2 wheels would spin. Thats why u need lockers, arb or detroit.

If only the front OR the rears are spinning while going uphill, there is a transfer case problem.

More likely it's what Canamark hinted at: open differentials will spin. Most likely when you see only the front drivers side spinning, then the rear passenger side is also spinning. This is because of the open differentials.

We've all see a car with only one wheel spinning in the snow (or the movie "My Cousin Vinny" :)); this car has an open differential. They could install a "posi", or positive traction (also known as trac-loc on newer Jeeps) that will apply power to both wheels if one starts spinning.

Take a look on Quadratec.com (and similar websites) everything you'll need to add locking differentials can be found there.

Not a t-case problem. Like fredjacksonsan said, it has open diffs. Get a locker, limited slip, spool, or anything like that and it will solve your problem.