Had to make a fast lane change the other day and when I hammered the throttle from a dead stop, the rpm swung over to about 2700 and the truck didn't accellerate all that fast. It also seemed to have a kind of "pulse" or shudder. I don't hammer it very much, just drive pretty sedately to save fuel. Just turned 149,000 miles. Don't tow very much and is usually under 5000 lbs.

 

If fluid is up to snuff, you just got first showing of failed tranny.