Ford 7.3L Power Stroke (1999-2003) :: Truck Wouldn't Go Fast / Plenty Of Power In Low End

2002 f250 7.3l diesel...

I got a new radiator and the tranny fluid was the worst the mechanic had ever seen. He serviced it and warned me that it might not be long for this world. I broke my flexplate and so I had to pull the tranny, and I decided to throw a new torque converter in because it was giving g me symptoms like a broken sprag( it wouldn't rev up or do a brake stand in gear, therefore slow takeoffs).

I was recommended to replace the tranny, I decided I could do the job again down the road if I needed to. About two thousand miles down the road I had a hard shift going from overdrive down to get up a hill and about 10 miles later the truck started slowing down....I couldn't give it enough pedal to keep it going and the engine sounded rough.

I pulled over and checked fluids and everything was fine, truck idled fine, bright red tranny fluid not burnt at all. Pulled on to the road again and it pulled fine until about 40 mph and it wouldn't go any faster. I got it towed home and ordered up a tranny from Ford figuring it must be toast, but I thought I'd throw it out to the forum to see if there is any other troubleshooting I can do before I install the new one in case it is some other problem or some electrical harness or something, or if that is a pretty standard reaction to a fried tranny.

Engine runs good at idle, shifts perfectly to drive and reverse, drives forward and back(I'm very hesitant to drive it out of the driveway in case I can't get it back in). Only happened two times before I decided to tow it. Temps looked fine on the gauges, fluid was a little overfilled in the tranny, but again, not hot or burnt.