OK, that is weird, some tests are in order, first you have to be sure the hubs are
working correctly.
Did you check the T-case lube? Was it low? Have you added lube to it?
Did you check the front diff lube? Was it low? Have you added lube to it?
Lock the hubs, drive in 2wd, is the problem still there?
Now unlock the hubs and drive in 4wd high, is the problem still there?
Your pending test results may solve the question.
I'm thinking there could be a problem in the front axle, like fried bearings
or a problem with a T-case bearing in the front output.
Wild