old RSMs siezed onto shocks, need clever way to remove

[zedfez]

If it's out of the car, you could try threading the nut partway on (just to protect the threads), flipping the whole thing upside down and banging it on something. You should probably use a board or something softer than the metal to avoid damage, but that would eliminate the problem of trying to hold the RSM while hitting it and allow you to use the mass of the whole thing and a hammer.