There's usually nothing wrong with "bad" Pantera rear axle bearings. What's gone bad in many cases is the required press-fit clearance between the mild steel stub-axles and the wheel bearing's inner race is gone. A few stub-axles were apparently factory-made too small so there was no press fit possible.
One person told me when she took her rear uprights apart, her axles FELL OUT! She went to a bearing store with her still-good stub-axles and found some sealed bearings that had ID tolerances that established a proper press fit. The needed press fit for the size bearings used in Panteras is 0.0005"-0.0008" interference. This fit is established by the bearing manufacturer, not Ford/DeTomaso.
So when a godawful-hard bearing race deflects under power due to improper clearance while running on a relatively soft steel axle that's slightly undersized, metal moves out of the highest stress zone by 'spalling'. You typically find stock axles that have 'wagon tracks' produced in two places on the axle where the bearing race(s) have made clearance for themselves.
Worse, the inner bearing actually runs on part of the splined area for the u-joint adapter driving the outboard end of the halfshaft, reducing the shaft area supporting the bearing. Not only does this weaken the hollow mild steel stub-axles, it causes the rear wheels to wobble while driving, wearing the expensive tread faster than normal and yielding odd handling.
I used to fix worn stock axles that hadn't yet broken by welding the wagon-tracks up with expensive Stellite-C (a cobalt alloy) then precision-grinding it down to the proper press-fit clearance. This gave a much harder surface for the bearing race to run on and reestablished a proper press fit. A few others hard-chromed their axle shafts to do the same. All this takes extra tooling for precision lathes.
This worked until the arrival of oversized rear tires made of far stickier rubber, and 3X stock horsepower street-engines. Then the repaired axles still worked OK in the bearing area, but broke just outboard of the outboard bearing where the shaft curves out to form the wheel flange. Brand new factory axles on LeMans racers in the '70s had about a 3-hr operating life. This was probably due to axle-flex and is a classic case of Smokey Yunick's "weak-link engineering" where you fix one weak spot and the next weakest point promptly fails.
The proper fix is Wilkinson's repro stub axles made of much tougher 300-M steel, which were also properly sized for a press-fit and were far stronger due to being made solid, not hollow. All the vendors now sell these stub axles. ALL long-term Panteras should be using them. They not only handle big, modern sticky street tires, they are successfully used with big racing slicks for high-power semi-pro endurance racing. FWIW, I no longer weld up and regrind stock 55-yr-old axles- a waste of time.