As for the idle, you have a vacuum leak.
Yes, after engine is warmed up idle should be 750-800rpm
IAC(idle air control) Valve is working because idle drops as engine warms up.
VSS signal goes to a few things and Speedometer is as the end of the line.
The Pink/Orange stripe wire on the VSS is the Ground, but it is spliced to 2 other Pink/Orange stripe wires, one for the computer and one for the Cruise Control.
At that splice this wire is Grounded to body or Frame, Speedometer needs this Ground to work
Other wire on the VSS is Grey/Black stripe, it is also spliced to 2 other Grey/Black stripe wires that go to computer and cruise control.
If you have cruise control test it if works and stays at the correct speed when set, if not then problem is in the wiring between VSS and the two splices, assuming VSS is good as you suspect.
Or if no cruise, hook up OBD2 reader/scanner and select VSS, then either drive truck and watch for steady speed, or jack up the back wheels and watch for steady speed.
If speed/signal drops or changes unusually then problem is in the wiring from VSS to splices.
If cruise works or OBD2 show steady signal then problem will be from grey/black splice to speedo or VSS ground.
Problem could also be in the dashboard itself, cluster wiring can fail.
From the Grey/Black wire splice there is just 1 wire that splits and goes to GEM and speedometer, so speedometer doesn't get Ground signal directly from VSS, see above.
Pin 1 on dash connector, Grey/Black wire, is the VSS signals "end of the line".
So for speedo/odo to work VSS needs to have a good Ground, and speedo/odo in dash does as well.
And speedo/odo needs the Grey/black wire signal from VSS.
Test Pink/Orange wire at VSS with OHM meter to Frame, should be 0 OHMs if there is a good Ground on that wire
Computer and Cruise do not need the VSS to be Grounded, they are connected directly to the Pink/Orange wire.
Last edited by RonD; 07-14-2016 at 10:38 AM.