96 GMC Suburban 4x4, reverse pop out

When I go in reverse under load in 2wd, trans pops out of gear. I have not tried it in 4wd. Under idle it backs fine. Forward gears, all is well. Any ideas?

When it "pops out of gear", what does it do? Does it slip out of gear, then go back into gear when rpm's go down - or does the gear shifter move - or ?

It would pop out of gear and make a high pitched grind sound. I would, quickly, put it neutral and wait a few seconds for it to wind down. I found out it was simply a broken rear snap ring. The OEM equipment was thin. The replacement is much thicker. I had local guys tell me it was electrical, linkage, "sunshield gear" and the best of all, TOTAL REBUILD. The luck of the Irish and $190 later we have reverse. Thanks for your interest.