clicking rear end

89 laredo, sometimes when I back up from a standing start the rear end sounds like a ratchet that is broken , sort of like its skipping gears , makes an aweful sound , but doesn't do it all the time and other then that is drives fine , except for the radical idiling

could be pinion bearings, broken pinion, worn clutch plates in the carrier (if you have a trac-loc), or it could be your t-case.....you should take the cover off of the diff and check things out. if you see a lot of metal its probably bearings.