I actually ended up pigtailing copper wires to each outlet (the one in the garage and the new regular 20 amp for the washer) from the aluminum feeder wires. I used those stupid expensive wire nuts filled with anti-oxidant goop and twisted both copper wires and the aluminum wire together firmly before twisting on the nut.
So now, the pigtail connection should be what carries the load, not the receptacle itself, and removing one receptacle or the other will not affect function or ground of the remaining.
The only thing the garage outlet will power is a battery tender -- it's just in the most convenient location for that and I didn't want to remove it. I can run the air compressor off another outlet.
Thanks for the tips.
You couldn't hear a dump truck driving through a nitro glycerin plant!
Badasses might screw with another badass. Nobody screws with a nut job.