VSS is at the top of the rear diff, you cant see it here because its on the front of the housing, but its right on top. If you follow the wiring for my diff temp sender its attached to the wires for the VSS as well...

 

yea, I just figured out that some jerk put a dana 70u from an 80's ford or dodge in the rear, I got the truck for 2,500 with a broken front axle and supposedly a little over 205k and I put a dana 60 up front and i'm about the get a matching 10 1/4 for the rear so I can have my speed/odometer/abs back. thanks guys, my stupidity, thought it came from the factory with the d70u

 

Correct me if I'm wrong here but though the VSS does tell the PCM and the speedo how fast you're going it's also what detects if you have locked up a rear wheel and does activate the RABS. So it IS part of the system.

To the OP has your speedo been working at all since you got the truck?