When tranny goes to 3rd gear the RPM's rev up and truck lacks power. If I put selector in 2 or 1 to prevent shift all is ok. Saw some info about Transmission output speed sensor, could this be the problem? 91,000 miles and never a problem.

 

Nope, when ya get that code, it means there is too much of a difference between the input shaft and output shafts. AKA slipping. The computer has a predetermined range of speed difference for each gear. If it goes too high, it is detecting slipping. It could be a bad 2-3 shift solenoid causing it not to go into 3rd gear properly.

 

Welcome to the site. With 91k on it, when was the last time you changed all the fluid and filters.

 

last tranny service was around 50k.

 

Not too bad. Are you getting any specific codes that help dictate the issue