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A few weeks ago I swapped rear ends from a Chevy II to early 70's Camaro. During the swap I noticed that the spring shackles moved a bit back & forth. I initially thought they were worn but the more I thought about it, I thought that since the truck was originally equiped with a torque tube this may have been by design to enable the truck to handle rough roads. If that is the case, since an open drive line is in place, what's the chance of the of the drive shaft being pulled out from the transmission if the shackles move? Will they move that much?

Robert your truck is a 3600 isn't it? They used a partially enclosed, partially open driveline that requires a fixed rather than pivoting rear axle mount.

The spring design should have pins on one end and shackles on the other to allow the springs to flex and provide suspension. The shackles moving is part of the design.

Provided the driveshaft is the proper length there should be enough of the yoke in the transmission to keep it engaged through the entire range of rear axle motion.

You can check this by jacking up the truck and letting the rear axle hang and then disconnecting the shocks to see if it falls so far the yoke slips from the trans.

Are you running a one-piece or two-piece driveshaft? One a 3/4-ton you might might to be two-piece with a center support bearing because otherwise your shaft might be pretty long. The longer the shaft the more critical its balance and the larger its diameter needs to be.

I think if you poke around in the tech tips area there is a write up by Grigg plus some links to commercial documents on driveshafts with some good information that might be better than what I'm offering off the top of my head.