2003 xlt 4.0 5spd electic transfer case.

when im in 4wd i get a loud clunking noise when i start off or when im turning left or right. it doesn't do it once it gets going unless i stop and start off again. it grabs and both front tires spin.

my question is what is causing the clunking. is it another hub bad or is it the transfer case itself?

 

update:

when i have all four tires off the ground i can't get it to pop in 4wd at all. even 4hi and 4lo