[silversup]

I really doubt it is the thermostat. Unless it is running too cool or gets real hot in a lil bit of time. I still say there are 2 possible things that are going on:

1st - the 3.0 is a slow dog of a motor. When you accelerate too hard at first u will get some speed then after a certain point of pushing the gas pedal, the RPMs will climb and you will not going any faster than the truck is trying to go until it shifts. If you take your time and acclerate the truck and dont go above 3000 RPM and you dont get a shift then you definitely have something wrong.

2nd- Tranny is just shot and nothing you will do besides getting another tranny will resolve this.

Take it for a drive and see what happens on the freeway. You will find your answer after a couple mins.