Thanks for the reply, and info. To be honest, I would not normally use 4x4 above 50mph, or so. But often, in highway driving up here, you may have bad, snowy spots, where it is really nice to put it in 4x4, and then the pavement clears a few miles later, and you want to go back up to 70mph, or so. Normally, I could shift back to 2 high on-the fly, at about 50- 60mph or less, and then keep increasing speed in 2high. Now, I can't do that - have to pull over, and shift into neutral, and usually reverse for just a spit second, before it will go back to 2high. Or, just accelerate to 60 -70 mph in 4high and keep trucking!
Did try changing the transfer case fluid, and it did not help!