if you from San Francisco Bay area, keep in mind that there are rules (California) for swapping engine in your truck. It has to be from a newer model and with all smog accessories.

 

Have you pulled the codes and asked the computer how it feels? While bad gas mileage in a pickup truck is a given, your engine is computer-controlled and there could be a problem it knows about - even without a Check-Engine light.

 

A engine that doesn't use any oil and doesn't knock or tick is not "tired". I am trying to read between the lines here, it sounds like you have a perfectly sound engine, but are disappointed in it's power output. If that is the case, then putting another engine in it that is exactly the same is not going to get you anything.

If you think you have a problem, you need to troubleshoot it. I haven't heard any real problems from you though, you just don't like the power output.

Is this a 4x4? Look on the tag on the doorjamb and tell us what axle code you have, and what tranny code you have.