Sounds like tcase to me. either way, it is not normal. have you installed a body lift? I would check to see if there is anything getting in the tcase shifters way that is preventing it from fully locking in or is putting tenstion on it when in 4lo making it easier to slip out of gear. Since it is only happening in R, your running gear should be torqueing in the opposite direction, so maybe something is binding?

'88 Raider. 2.6L dead...2.4L swap in progress, AT, 33's, 2 bouncy seat,AC, CC, 2.25 exh, 2" Tbar, 2" BL, rear DB, rear LSD,4.90's, SW hubs,gen2 uca's, KYB shocks, snorkel, elec fan, MSD.

'99 chev k3500 crewcab
'15 Subaru XV
'03 BMW i325