Does anyone know if there is a trailing arm from another Subaru that will fit in place of the stock one on an 86 Brat? I know the part number is 721070130. I've tried everything and can't get the ring nut free and I doubt the bearings are going to come out if I do...

 Wagon or sedans 4wd from 80-84 will fit.

Hatchback til 89

You really just need the right tool.  Chevy spindle nut tool with 2 teeth ground off will fit it.

721070130 is found on 80-81 4wd wagons, 82 4wd wagons and brats,83/4 non turbo 4wd wagons/brats and  presumably newer brats.

Other 4wd models have different part numbers for some reason(lighter gauge metal?)

Air chisel for the win.