Well, I can safely say that I am greatly relived that your theory is wrong. The drums are keyed fine and very securely attacked to the axles.

However, there is an issue back there and it some ways it could be far worse.

The play and thunk I hear is from my lovely 4:10 differential. I removed the cover and changed the gear oil and gasket (no metal shavings) but found that I could turn either wheel or the driveshaft and get noticeable movement before the gears engaged. I took a look at the differential gears and saw no serious wear, but the pinion gear may be worn. It was hard to tell back in there. but I think I see some wear on the ends of the gear... Then again, maybe it was something else.

I could be lucky that the big nut on the pinion needs to be tightened, or not so lucky and that pinion gear's days are numbered. I'm not sure and won't be until something breaks or I get a chance to drop the axle and investigate further.

the differential bracket bolts were nice an tight, so I'm at a loss as to how it would have so much play in it without generating some metal debris in the sump of the pod...