Since you wiggled the harness and things got better, maybe the problem is there? Around '93, the main transmission connector was changed and most trucks were upgraded to the snap-on connector (vs. bolt-on for the original design). The vehicle harnesses that I have seen with the upgrade were spliced in with butt connectors about a foot from the trans connector, if yours is one of these, maybe a butt connector is going bad?

If it is a VSS/DRAC problem, make sure your VSS wiring isn't damaged, I think almost half of the harnesses I look at have bare wires showing near the VSS connector. I would think you should be able to swap in almost any DRAC for troubleshooting purposes to see if that fixes the problem as well. I have also seen a couple of DRACs with burned traces on the PC board and that's easy to check for. If you come up empty looking for a replacement DRAC, let me know, I might have the correct one.