'98 12 valve with auto. When I first start up truck it ran fine, but after it heated up the rpms started to rise at idle. Had to shut down to avoid going over 2000 rpm in park.

possible causes?

 

A little history please. Just buy it? Problem start suddenly or been slowly building?
Common for a cold diesel to idle slow when cold and a little faster warm, thats why you adjust everything with motor warm.
I guess the first thing to look at is the return springs on the linkage, and the throttle cables.
Have you done any mods? What about mileage?