[AussieinNC]

Originally Posted by HO455

As it left the factory the 1/2 ton coil spring suspension has two solid shafts ( i.e. no sliding spline). When the axle moves up and down the drive shaft has to move back and forth. The yoke on the output of the transmission slides to accommodate this movement. The factory carrier bearing's rubber bushing holds the bearing in position vertically, side to side but allows movement fore and aft as needed. The aftermarket bearings I have seen seem to allow much less movement fore and aft than any factory carrier I have seen. Admittedly I have not found a NOS carrier to inspect.
When you lower the truck the range of movement of the shaft is shifted forward from the factory range of movement. The factory bushing sometimes will accommodate the new range of movement. At other times it fails from operating at the limit of its forward range of movement. And of course it's old. When the factory bushings go bad is is usually noticed as a vibration in the truck as the rubber is no longer able to hold the bearing in vertically and/or side to side, not a catastrophic failure of the mount.
Replacement carriers are designed to do the job as cheap as possible and no GM specifications are used. Thus the reduced amount of travel that they seem to allow. (And don't get me started on the "Engineered to Fail" traits of modern day rubber products. Like rubber bands).
Due to tolerance stacking and how much the truck has been lowered sometimes the replacement carrier will survive. The trick of making sure the new carrier is as far forward as possible will work on some trucks. On other trucks the shaft is too far forward and when the carrier is bolted down the bushing is preloaded at or close to its limit of travel. When the limit is reached the carrier mount has to flex and eventually it breaks.
The same thing will happen if you use a aftermarket billet carrier with the stock 1/2 ton coil spring driveshaft. The billet carriers have no provision to allow fore and aft movement and they require a slider in the rear shaft.
On 3/4 ton and leaf spring trucks the carrier bearing is different and the rear driveshaft has a slider just behind the carrier. The carrier holds the front shaft in one place and the fore and aft movement is taken up by the slider in the rear shaft. There is very little movement of the yoke at the output of the transmission.
The photo shows the factory driveshaft I removed and the new one with a billet carrier and the slider in the rear shaft.
I hope this helps.

Such a perfectly presented explanation....agree with 100% of the above...but if you want a one piece shaft, follow the shaft builders measurement process to the letter....