why would my truck run just fine then start to idle rough?

It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. You may have an electronic idle control problem, a sticking EGR valve, a vacuum leak, or many other things. Again basic tests need to be done, your car has on board diagnostics using a scan tool may quickly identity a problem if the tool is capable of reading live sensor data.

That's a tough question. It could be many things. I can tell you what I would check first. Is the ignition system in good working order? Is there a vacuum leak? Is the IAC Motor working correctly ? When it's running fine is the idle high ? I would recommend that you fine a good shop that you can trust and let them take a look at it