brakes

I think your rear brakes are not working at all and when in 4WD, the rear wheels are directly coupled to the front and it's the front brakes stopping the rear wheels because the front and rear drive systems are connected through the transfer case. You could have a failed master cylinder or brake proportioning valve which controls the brake fluid pressure at the rear brakes. Here's a directory link for you: http://repairpal.com/jeep-repair-in-archbald-pennsylvania

They don't even work then, they're just being stopped through the drivetrain by the front brakes. Jegs sells an adjustable proportioning valve. You have to remove or gut the old valve and replace it with one that is adjustable. Then your back brakes will work. I did this to my Wrangler and it's stopping fine now. Just don't over-pressure the rear brakes or your wheel cylinders will start leaking.