Wheel bearing..

Seems I’ve got a rumbley wheel bearing on the rear.  Is this a knock it out at lunch job? Or is it a world or pain job? Do I need to drain the diff?

It's somewhere between the two. As long as you jack up the axle casing so that the hub is above the oil level filler plug, you do not need to empty the diff. oil.
So, jack up the axle casing, just enough to stop the oil from leaking out, remove wheel and brake drum. remove brake pipe and 4 nuts securing the axle to casing. Either plug the brake pipe or use brake hose clamp to stop fluid from escaping. If you have slide hammer, use it to remove axle shaft. If not, replace drum the wrong way round and use it to pull shaft out. The bearing is held onto the shaft by a lock ring. Using an angle grinder, carefully grind away sufficient lock ring material to nearly cut through to the shaft and finish with a chisel. Be careful. Do not cut through and damage shaft. Old bearing should be easy to remove as it is not a tight fit. If it wont pull off by hand. lightly clamp in vice and tap end of shaft with copper hammer. Do not use steel faced hammer as this may damage splines. You can use steel hammer but place piece of wood on top of shaft. Clean everything and refit in reverse order.
It can be a bit daunting but it is not rocket science.
Hint.
Put bearing end in bucket of ice colt water. Prepare brake plate and assemble onto shaft after drying shaft. If bearing comes separate to lock ring, warm bearing in micro-wave and slide into position. Heat lock ring in boiling oil and slide into position. I have a piece of scaffold pole that I use to hammer lock ring into position. This tube must be really clean so that you do not drop anything onto the bearing.

Roger

The lock rings are different for ABS & non-ABS, the ABS type lock ring has the teeth on it.
There is an oil seal to change behind the bearing inside the end of the axle tube, the seal lip rubs on the lock ring, so check that the surface of the lock ring is polished, I had one that was left unpolished once, and it leaked until I put the whole thing in a lathe and polished it.

If it is a non-ABS car you shouldn't need to bleed the front brakes, or to touch that nipple on the compensator. (It is for the front circuit)