2004 stx jerks in 4WD?
 

I have a 2004 f150 STX 4x4 with a 4.6V8. I have checked out everything and can't find the problem. will I drive the truck in snow in 4x4 drive ,the whole truck jerks forward and backward on ice or snow. even speeding up to highway speed it still does it. both front hubs are working correctly. Two front wheels will spin free in 2wd engine running and drive OK in 2wd. They both lock in with 4x4 drive, engine running. Checked u joints, they are OK. Any ideas???