hmm... a clicking noise could be just aobut anyhting from a stick or rock caught in a wheel, to a worn axle or if you have CVd's a worn drive cup, to a broken tooth on a diff, to a chiped tooth on a tranny gear. start by looking everyhitn over really well. Pull off all the wheels and be sure its not something like a small stick or rock causing the sound. Then put the wheels back on and roll the truck around listening for the click. if its from the front or rear its probably a diff if tis from teh cetner its probably the tranny. if you still can't find it remove the tranny and rotate each end of the truck and the tranny seperately to further isolate the source of the clicking.

I hope this helps.

john