When in gear it won't idle, runs from 900 rpm down to about 400 rpm. It is an 03 automatic. Just started this as the weather warmed up. Is this a vacuum line issue somewhere? Computer or a sensor?

 

After engine is warmed up put truck in Park and leave it idling, open the hood and unplug the IAC(idle air control) valve, engine RPMs should drop down to 500 or engine may even die, either is good it means no vacuum leak.
If idle stay above 700 then there is a leak, easier to track down with IAC Valve unplugged.

Also Clean the IAC Valve, it can get sticky inside so computer can't set stable idle.

Always clean MAF sensor every two years or so.

 

This probably isn't the solution to your problem, but speaking of vacuum line issues there is a design flaw in the PCV system. The rubber elbow coming out of the intake just behind the throttle body will slowly destroy itself over time. I replaced mine at around 100k and it had almost split in half. You can get a replacement from the HELP section at any auto parts store, just get the Ford diesel truck one, the one that's the same at both ends, and borrow their hose cutter to match it to size. It's like 8 bucks.

Improvement in performance isn't really obvious, but it's nice to not have a hole in your PCV system that could manifest itself later as a bigger issue.

Also if you have your original fuel pump I would test the fuel pressure, that's what ended up being my problem when I had issues similar to this.

edit: I continued browsing the forums and found a topic on this issue, sorry to bring it up again, I had not seen anyone else mention it before!