I had that problem with my truck ('97 5-speed 4x4 w/3. 54's). Very annoying. I knew someone with the same year truck, same gearing, etc. , and their cruise control worked fine in 5th with the GV on. My cruise control would work okay in 5th without the GV on, or 4th with the GV on, but not 5th with the GV on. I called Gear Vendors and they said they were aware of the problem, some Dodges work fine and others don't, and they don't have a fix for it. They said they'd put me on a list of people to contact when they figured it out.
Last summer my GV unit let go with a bang. My solution: I had it removed!
Andy