shudder in 4wd

Joined: 17 Jan 2008 Location: A.C.T

i'm helping a mate with his car. its a 99 wrangler tj

it shudders really bad in 4wd full lock but not with the wheels straight or ever in 2wd. i thought it would be the the sun gears are stuffed.

pulled the front diff cover off and every thing in the diff if fine, gears nice and smooth, no niks, no breaks.

could it be back in the transfer case? has any one had this problem before?