M1028A1ShelterCarrier,
To be clear - are you rebuilding this one, or replacing the whole unit? I remember you mentioning in the past about getting just a seal kit.
I ask because every reman unit that you will buy from an autoparts store (that I have seen) will already have the backing plate on it. So there is no need to take off the retainer ring and the 'squared' nut. The reman units from Autozone & Advanced Auto have the backing plates. Heck, even getting one from the dealer ($650!) will have the backing plate. Here is a pick of the one that I got from AutoZone.
However, if for some odd reason that you are replacing it and the new unit didnot come with a backing plate, you can take it to a autoshop, from where they can use a air-punch and get it off for ya. I don't think it's really possible with just household based tools. I have an extra one that I can send you, but it would take a few days to get to you.
-graham
86 M1009