So here's today's installment of the drama:
I pulled the axle on the side that was leaking (checked the play in the axle first and it was fine - and on both sides, so I know the pins in the Sure Grip are in place). Inspection of the bearing shows it is fine. I talked to the guys at Randy's Ring and Pinion and they confirmed a few things I saw folks offer here - the differential lube should never reach the axle bearings except in minute amounts. The seal in the housing is what separates the diff lube from the bearings. I KNOW!!! - how are the bearings in my other two Darts that have been untouched for 20 years or more working fine with no service!?!? I clean and repack the front wheel bearing every year or so.
According to the RR&P guys, even though the seal feels pliable and shows no damage, that doesn't mean it's OK. You can't feel or see if the seal has become out of round or worn so that lube slips through. So today I replaced the seal, repacked the axle bearing, replaced the gaskets and put it all back together following the service manual procedure for axle end play. I replaced the soaked brake shoes, and that meant replacing the other side shoes to match, adjusted them all up. Tomorrow I need to drive 50 miles or so, put 'er up on the lift, say a few prayers and pull that drum and see if the diff lube stayed where it belongs. Wish me luck!!!
Thanks to everyone that contributed here, even after 35 years of Darts with 8&3/4 rears, I learned some new stuff! I'll let you know if the fix worked...