T.Low-
Did you happen to increase the size of your tires prior to changing the t-case? Was it even worse in the sand with bigger tires and the factory AWD set up?
Exactly. Yes I did. I only had the van for a few weeks before lifting and adding tires (and was away in Mexico or home driviing my F-150, so I didn't experience the completely stock van very much).
But yes, the 15 miles from Westport to Tokeland via the beach was with stock tcase, stock 3:43 gearing, and 255 70 r 16 AT's. Soon thereafter I went to 4:10's, and then a tcase swap.