Interesting. My theory is that the shifter would not move much when the transfer case moves from engine torque. The shifter is rigid mounted to the body. I contend that the input selector gear was just a hair shy of engagement. Then it partially began to engage into 4H when the engine torque under downhill conditions moved the txfr case but the shift linkage remained in place.I don't think a bad engine or transmission mount could change the shifter position. The shift lever is bolted to the transmission/t-case, so any excess movement of the engine/transmission, the shifter would move with it, not pull it out of gear.
I am running out of ideas....
When I ckd it with a friend at his shop we found the shift linkage to be approximately 1/2" off from correct factory settings.