Chevrolet Repair: 4l80e trans, and 6.2 timing., vehicle speed sensor, speedometer gear

Question
I have a 90 Chev 3500 with a 6.2 and 4l80e trans.  Just rebuilt the engine, bought the truck in non running condition, but trans was supposedly recently rebuilt.  The service engine light is on.  Took it for a test drive, the speedo doesn't work, and it doesn't want to shift out of 1st. If I let off the accelorator it seems to shift into neutral, I say this becasue if press on the pedal again it will rev almost freely, and if I pull the lever into 1, it will downshift.  Any idea what the problem might be?
What should the timing on the 6.2 be set at, and how do i check it, I know how to check on a gas(timing light), but not on a diesel.
Thank you  

Answer
Hi Jimmy,
The timing is set by loosening and rotating the injector pump to lign up the tiny little mark on the top sloted mounting hole, with the mark on the timing gear cover right beside it.

The other problem may be associated with the vehicle speed sensor on the tranny. Looks kinda like a speedometer gear, but with wires in stead of a cable.

Van