I abuse my trucks way more than most, and I've twisted driveshafts, torn bearing pillars off the crossmembers, twisted axleshafts, smoked clutches, broken trannies... ... I know what NOT to do!! :-laf I've also owned class 8 trucks since I was 18, and it's often said I grew up between two frame rails... . I've broken more than my fair share of driveshafts, and even had some come up and greet me in some cars I've had. For the aluminum shaft to break, I can't say as I'm surprised..... the slightest weakness in wall or weld will give. Now, figure into that, that you're actually pulling on the driveshaft... ... "What?" you ask? Well, he's in reverse, right? Uphill? The slip yokes are binding under pressure and torque, not readily giving or slipping, yet the rear axle is rolling "away" from that yoke from torque yaw/spring wrap, whatever you want to call it, as the truck is pushing it's load uphill, with the axle moving under it. It is literally pulling/pushing the shaft into. As the slip yoke advances, it instantly has to retreat if the axle rolls back to it's assigned position. This stress is causingthe twist at the output shaft. Even the steel shaft is encountering this problem. Now, when you go look at a class 8 truck, you'll notice there is no slip yoke in the back of the transmission. It's built into the driveshaft, and is massive, with yokes directly bolted onto the end of the output and input shafts. Why is that? Hmmm, maybe they know something that light duty manufacturer's don't

..... or maybe it costs too much to build something that'll last 800k miles!!!
While I don't think it
should twist off, I can see how it would in 4 low, reverse. Locking the hubs would probably aleviate this problem, as it would prevent the axle wrap from being so severe, as well as any torque hammer that may happen from slippage... . It's simply overloaded for it's poor design. Engaging the hubs will distribute torque more evenly, allowing
both axles to push the truck, and the load, uphill.
And for what it's worth Mike (not much I'm sure!!!:-laf), I've pulled more-than-I-care-for driveshafts in muddy fields and many barditches, trying to get 100k+ loads of grain out during harvest!#@$%! Nothing like a couple good 1/2" chains and a Versatile tractor..... and laying on your back, with the water rising up your back... . or a plethora of grassburrs to lay on in the sand... ... I still carry a large piece of cardboard under my mattress in the sleeper... . there's also some 1810 R281X Meritor U-joints back there, too.
