My '92 SR is having 4WD issues.

The truck is great in 2WD. If I shift the transfer case to 4wd hi there is much wailing and gnashing of teeth.
The same if I go directly to 4WD Hi Loc.

But if I go directly to 4WD Lo Loc 4x4 works as advertised, I can then go from 4WD Lo Loc to 4WD Hi Loc and all is well. But if I go from there to 4WD Hi with the open diff, there is more wailing and gnashing of teeth. Crunchy noises and vibrations.

Any ideas what the problem could be?

92 Montero SR
84 Maserati Biturbo
89 Mercedes 190E
77 Fiat X1/9