Haven't had a chance to check the ATF level. Last oil change (3500 miles ago), they said both the tranny fluid and the transfer case fluids were just about ready to be flushed and replaced. So, I'm hoping that that's all it is.

As far as slippage, there is none. Just the transition between 1st and 2nd is not too smooth. It has a tendency to almost 'jump' into second gear and lurch the truck a bit.