Ok, so when I engage my 4x4, the front end starts pulling me side to side, not consistently, but I will get a random jerk to the left or right every couple seconds. When I turn it off it's fine, it drives straight. Whats going on?

 

What year is the vehicle?
Manual shift transfer case or electric shift?
If electric, does it have 3 or 4 buttons in the dash switch?

 

Bad CV axle, maybe? Oil in the front differential?

 

1995 GMC Jimmy. Its an electric shift but the vaccum for the diff went bad so now the diff. is manual and I know it engages. T-case is still electric, 3 buttons: 2wd, 4hi, 4lo. I had new seals put on the diff. durring fall and worked fine, then in winter is started doing this, about a month ago.

 

If the front differential is original, it applies power to the wheel with the least amount of traction, and that might be all it is... (transferring back and forth).
If that's not it, several other things could cause it:
Worn or loose steering linkage or suspension parts.
Bald tires.
Wheel alignment out of spec.
Worn front wheel bearing/hub.

 

I had this issue with my old truck... when accelerating in 4x4 it would pull to either direction depending on steering input... didnt do it in 2 wheel drive as you mentioned.

Replacing the pitman and idler arm solved most of it. I later added a steering stabilizer and the problem disappeared completely.

My advise would be to go through your front end / steering components and replace anything thats worn + get an alignment.

 

Must be the alignment because it needs one. I'll check into all you have stated, Thanks everyone!