Alfa rear wheel bearings have a fair amount of side play in them, even when brand new. If you don't hear bad noises at speed, or when cornering hard, I wouldn't worry about it, unless the play is extreme. The rumble certainly could be the bearings, though generally you'll hear the bad bearing louder when turning a corner away from it (i.e., left bearing makes more noise, under load, in right turn). There are plenty of other things to make noise/vibrate/rumble in the driveline though.
Front wheel bearings are adjustable and tapered and can be adjusted down so tight that you can't turn the wheel. You do want a tiny amount of play at the front. Rear wheel bearings are sealed units and cannot be adjusted. So I don't think it's a useful comparison from front to rear wheel bearings.
You don't have to change the oil seal in the axle tube when changing wheel bearings, but it's generally a good idea. 2000 have a drive-on retaining ring for the bearing; up through 1750 have a screw-on ring with unobtainium lockplate.