I'd taker a very close look at the universal joints and the rear end pinion bearings before digging into the transmission. Coasting with the clutch disengaged takes the load off the entire drivetrain, including the rear end and driveshaft. An SM-420 is about as bulletproof a transmission as there has ever been. The rock crawler crowd loves 'em because of that super-low ratio granny gear, so the prices are getting a little steep on good condition used transmissions.
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!