I have a 08 chevy 2500 6.0L, it runs great except when im doing 50ish + MPH i can shift to neutral and the RPM's hang around 1800-2000 even when i brake all the way to a stop. I can brake with it in drive and the rpm's fall normally most of the time, which is why this is an issue because the times it doesnt it becomes a hard shift and spins tires taking off. After checking with my modis i found the TPS 1 and TPS 2 didn't match, so i changed the throttle body and it relearned it immediately and the idle sits around 600 however the issue still remains when shifting to neutral at speed and coming to a stop. On a rare occasion it will hold a high RPM when sitting still at idle but its very rare. I have checked for vacuum leaks with no luck, Not setting any DTC's I've also probed APP and found no issues with it, and now no issues with the TPS either. quickly becoming stumped

 

Probably a bad Idle air control valve (IAC). Easy fix if so.

 

You said that you ckecked vacuum lines.
This can be tricky if you did not take your time doing this.
I would double check inche by inche again.
Any vacuum hose big or small that not connected firmly can affect the idle.
A bad PCV can also make a high idle.
Your truck's PCV Valve is on valve cover.
If your PCV Valve is not working properly.
What i have done in the past with the same issue is to remove the valve's cover and clean it. That fixed my problem and saved $90 because that is they go for.
If All looks good on your engine, Disconnect the Battery over night.
Gook lucky