I believe it is part #2 on the diagram below (assuming you have the 10 speed auto).
P2683 is defined in the service manual as "Engine Coolant Bypass Valve "A" Control Circuit High". I could not find any testing procedures for this, but the code would indicate that the valve is not operating when commanded by the PCM. I don't have any other details from the manual on the code, but from what I know about how these circuits operate, my guess is that the PCM is sending the signal to operate the valve but is not seeing the voltage drop when the valve is commanded to operate, so the PCM sets a "circuit high" code. Faults like this can also be caused by open circuits, so I would not launch the parts cannon at the coolant bypass valve without confirming failure since this could also be a wiring problem. My guess is it should be fairly easy to test - probably just a simple 12V solenoid. Experienced people feel free to correct me if I am wrong.
Description from the factory service manual:
The transmission fluid cooling system for the 6.2/7.3 V8 and the 6.7 diesel engine consists of a heat exchanger mounted to the transmission. The
heat exchanger has both transmission fluid and engine coolant flowing through it in separate chambers. The transmission fluid warmer has engine
coolant flowing through it to warm the transmission fluid during the engine warm-up period. A coolant control valve attached to the coolant return
hose allows coolant flow through the warmer when the transmission fluid is cold. At a predetermined temperature, the PCM signals the valve to
close, shutting off coolant flow to the warmer. The transmission fluid bypass valve on the warmer directs transmission fluid either back to the
transmission or to the cooler depending on transmission fluid temperature.