I agree with all the votes for axles and gearing first. But also, if the motor is tired and the truck still doesn't seem to perform.... Replacement is an understandable upgrade. Especially here in Colorado at this elevation. Motors starve enough up here at 7,000 ft. My suggestion is something not too exotic. maybe just a fresh 350. More power, more breakage. I'm sure you'll be able to find some guidance and help here (on CK5). Post lots of pics and questions. Lots of knowledge on here.