i have a 99 foreman 400 4x4. when i try to pull something my front tires turn but my rear do not. i tried to go over a small dirt mound i went slowly to see what would happen once the front tires got on top of the dirt mound i gave some more gas and the four wheeler felt like it was going to buck me off like a bull. when i drive straight and normal the atv works great

 

I'd be looking at the rear U joint and drive shaft.

Put it up on a lift so you can turn the rear tires and see what broke.

 

Have had an occasional similar post in the past! In one case, the splines on the drive shaft had gotten water in them, rusted out and were weakend, and finally just stripped.

Start by checking that out. I would put it up on blocks and run it, then apply the rear brakes, and see if you can tell what is still turning and what isn't on the rear drivetrain.

Good idea to grease up those splines now and then, especially for those who get into a lot of water.