So lets be clear on the symptoms.
When you attempted to leave the intersection the truck revved up as if it was in neutral, as if there was no load at all?
The truck revved up as if you were in gear but had your foot on the brake while pushing the gas pedal?
If the former, it sounds like the torque convertor was not enabling or the trans was slipping badly in first gear. I suppose it could have been the transfer case not entirely changing back into gear when you went from 4wd back to 2wd.
By engaging 4low you forced the tcase to change and attempt to go back again. I sure miss the days of the transfer case shifter when you knew exaclty what gear you were in