IIRC, you run W-5s on a 3.79" shortblock. How about a complete
run-down on the engine and vehicle?

I'd have to guess, you're running out of engine in high gear.
You are probably getting the tires up to a speed where they
are cleaning themselves pretty good and the added 4" width
per tire is getting more traction than the engine can pull the
truck.

We need a mud-slingin' smiley.