03 H2 - shifting problem

[vajimbo]

Was on a road trip last week, cruising along at 75 MPH when the truck suddenly downshifted into 3rd. CEL came on as well. Pulled over to check things out, but couldn't see any problems.

Started out again, and it started in 1st, shifted to what appeared to be 3rd, then downshifted for no apparent reason. Stayed there for 5 secs, then back to 3rd where it stayed. Had to run 300 miles with the truck in 3rd.

Other symptoms - computer is not picking up anything - Odo and trip computers not registering anything - e.g. avg mph.

I was thinking it might be the Vehicle Speed Sensor. Just replaced that and no change.

Any thought on what else to check would be appreciated.