This problem has been getting worse and worse for at least a year. If I accelerate quickly or the engine is under load (like going up a hill or on the freeway) or if it's hot out, the speedometer will jump around like crazy or stay at 0. One clue is that it affects cruise control. If I'm using cruise control and it does this, I can feel it try to adjust. It gets pretty violent if I'm doing 70mph and the speedo starts jumping down to 35 and up to 90. Sent the DRAC module back to ACDelco to be remanufactured and it didn't fix the problem.

Lately it got to the point where it reads 0 mph about 90% of the time. I replaced the speed sensor on the transfer case and used electrical parts cleaner on the connectors. I'm getting numbers on the dash again but they're still erratic. I noticed that the speed displays perfectly going in reverse and while decelerating. If I give the gas pedal any amount of pressure, the numbers start jumping around again.

To recap: I've replaced the vehicle speed sensor, had the DRAC remanufactured at the dealership, cleaned the connectors under the truck. That basically got the speedometer to jump around more and read correctly in reverse and while decelerating. What's left to do? All I can think of is remanufacture the ECM but I don't even want to think about the cost on that.