truck starts and drives but won't get above 25 mph. What is wrong?

have it ck'd for free from aamco if poss trans related and they will scan for codes with a prof scanner

SCAN TEST is still needed! Regardless of light!

There several possibilities of what could be going on, all of which require diagnostic equipment in order to help pinpoint the problem. Shotgunning parts into a vehicle can become very frustrating and expensive if you're not luck on the 1st round. I would recommend to have a shop that you feel comfortable with , look at it for you. In the long run will be much cheaper.

My brothers dodge did same thing. His catalytic converter was clogged. Replaced all is good.