Truck runs fine, idles fine, accelerates fine, starts fine. Once its warmed up and idling and I try to shift into reverse or drive then the rpm will dip badly and the engine will hesitate. It's a 2002 3.0 automatic.

 

On the drivers side of automatic transmission is the DTR(Digital Transmission Range) sensor
Its on the shift rod/arm

This tells the computer when you shift into gear and at that time computer should open IAC(idle air control) Valve a little to increase RPMs by 50 to 75
Computer will also increase RPMs if AC is turned on, working or not, turn selector to OFF and RPMs should go lower, then turn to AC and they should go higher
Computer also holds RPMs higher on Cold Start, 1,000rpms or so, and then slowly lowers RPM as engine warms up
These all rely on computer and IAC Valve working
If the last two also don't seem to work then clean the IAC Valve

If its just the "in gear" that's not working then unplug the DTR sensor's connector and look for corrosion, if none is seen then plug it back in, this unplugging and plugging back in cleans the pins and slots
Then retest if computer is "seeing" the "in gear" status from DTR

DTR also controls Reverse Lights and Starter Motor activation, if there has been no trouble with these it would be hard to blame the DTR but still could be bad contacts inside

 

Thank you! I'll give that a shot.