Not fully fixed yet. Ordered the rear drum bearing kit, upper felt seal, and the blower motor seal. The repair went fairly smoothly. My wife held the ball shaft onto the drum while I screwed it in, which worked much better than taping it to the drum. Rather than use a wire brush to remove the seal adhesives, we found Goo-Gone removed the adhesive easier (just be careful to not get it near other glued parts in the dryer). Most of our time was spent removing lint and more lint from all the nooks and crannies (could have made a sweater with it).
While all the parts did need replacing, I now realize the hum and rattle seems to be from a motor bearing wearing out. I used a liberal amount of WD-40 to displace/wash out the lint and grit from the rear motor bearing, then followed up with a couple of drops of 3-in-1 oil (with the dryer tilted forward to let it ooze into the bearing/shaft area), then wiped the excess oil up. The hum is MUCH quieter and the rattle seems to have stopped for now. I will probably order a motor once this temp fix gives out...the motor installation seems to be even easier than the rear drum bearing.