I've got a buddy with a recently tuned up99 2500 4x4 that can only reach a top speed of 50 mph. The 5.9l gas engine starts fine, idles fine, does not hestitate during acceleration. I think the transmission is going but want to know your thoughts?

 

be more specific. what happens at 50 mph? does the truck rev through the full rpm range with no problems? does the trans feel like its shifting properly? are you towing 45,000 pounds through mountain roads? details, please