There is a splined drive plate in the ring gear carrier that is engaged by a sliding gear to select high range. That plate and gear are notorious for getting worn in such a way that the rear end will shift into high but as soon as a load is applied it disengages and goes in to a "false neutral". As soon as the driver backs off the power, it shifts back into high range. The only fix is to replace both items- - - -they wear pretty much equally. It isn't a terribly expensive fix, but it does require a full disassembly of the differential.
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!