It's a rebuilt unit? There's a possibility a replacement gear, either the cluster gear or the 5th. output gear, has teeth that are not ground at the correct angle. It would be necessary to know what parts were replaced during the rebuild to make a more accurate diagnosis, or a teardown and inspection.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!