Transfer Case?

I have a '97 Jimmy AWD automatic w/ floor shift (not 4x4-no switch). Sometimes when accelerating I hear a loud clunking noise and the truck jumps a little. I suspect it is the transfer case. I climbed under the truck and was able to move the front axle a little back and forth. While moving the axle I heard a noise coming from the transfer case. Does it need to be replaced? If so, is it a job I could do or does it need to be performed by a mechanic. I have moderate mechanical skills.

Thanks!
dru

Have your differential and tranfer case fluid levels checked.