Without knowing exactly whos shift kit and the part number, it's a guess. But the govenor would be my first suspect. Since it's in a truck you may be able to remove it and see if it's moving freely and not sticky. You could even replace it with another and test the operation with a known good unit.

The modulator can over-ride the govenor to control down shifts based on engine loading (vacuum), but the govenor is the primary shift controller based on driveshaft speed.