It's also possible the needle bearings on the cluster gear are bad. In 4th. gear, there's little to no load on the cluster gear, and the niose of a bad bearing is minimized. Time for a rebuild, before you chew up another bunch of expensive gears. A bearing and seal kit isn't that costly.
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!