4WD Lo but NO 4WD Hi?

I'm having the opposite problem of poster "hunter" with a '90 K1500/auto/V6: although the 4WD light comes on in both ranges, I have
front axle pwr in Hi and not in Low; had to spend a little time in the sand to verify. Anything in the xfer case that could cause this? (Yep it goes into low ratio for the rear.) FWIW when really spinning up in 4 Lo (when I attempted to spin the fronts), I get a fugly loud whirring too, just to make it more ominous.
Much TIA for guidance, or even knowledgeable wisecracks. ;-)
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