Wow. How much weight did you have on your work truck? Was it one of the older GV units? I noticed that for 'commercial' use, GV rated there unit at less then "recreational" use. Heck, they even have that extra oil capacity pan/cooler. The limit it to 25,000lbs GCWR vs the 30,000lbs for recreational use.
http://www.gearvendors.com/commercial.html
From there site:
Maybe backing up killed it.....
When I had my GV installed, I was wondering about the GM oil that they recommended. My local parts store noticed that a Royal Purple brand met the GM/Mopar specs, so I bought that. I don't know how much the GM/Mopar stuff runs.....
Maybe the era of the GV OD is going. I mean, most trucks have OD now, if not they have double over (well, the NEW, NEW trucks, the 5 and 6 speed auto's). Heck, they're making a pickup box for the F450 truck for 2011(?) now! In the past, most people were towing GCWR below 20,000lbs with transmission with out any OD at all (be it stick or auto).
Side note: My GV isn't that old. It was put two (three?) years ago, and I doubt I have 10,000 miles. Any how, if you have a unit w/ the mechanical speedo, double check their little extension cable. My drive cable broke where it comes out of their GV adapter. Fortunately, a local shop made speedo/tach cables, so it was quick fix. When I took it apart, I noticed that it was DRY. VERY, VERY DRY. I think it was not lubed prior to install (maybe it was the dealer I had do it.... it was a GV dealer, but I bought the complete unit off EBAY from another dealer out of state....) I guess it was binding and eventually it was more friction it could handle.... Just a FYI ;p
Edit- Prior to it breaking, the unit would act funny. Sometimes it would shift to OD below 25MPH! I think what was happening (just a guess) that the cable was starting to come apart. The friction was making it bind up, the cable would twist and then it would "release like a spring" spinning the cable and tricking their electronic unit I was above 25MPH even though the speedo didn't indicate it. Just a crazy idea..... so far, it hasn't shifted into OD below 25 (or below 40/45MPH where it should shift).