Ford Super Duty (1999-2016) :: 2002 F250 Drives Unusually Rough When In 4 Wheel Drive

I purchased the truck used last summer and this is the second time I've put it in 4wd. The first time was over the summer, and I had a hard time getting it into 4wd, and it made a "clunk" sound initially but drove fine, then went back into 2wd just fine and had driven fine ever since. Today I put it into 4wd, no clunk or any other noises, very smooth, but it drove very rough, almost as if the wheels were out of sync or something. It seemed almost like the wheels were breaking on and off on their own, very weird. When I put it back into 2wd, it did that for a few seconds until it went away. I only drove it in 4wd in my parking garage as I didn't want to risk any more damage to it on the road.