Thanks for the info, 3B. That helped get me on the right track. It was also quite a surprise, and a testament to the "you really don't know who replaced what with what" on trucks that have been around for a while.
Using this info (and the interwebs) I believe this is a 14 bolt GM - possibly a a Full Floater with a 10.5 Ring Gear - and from 1976. If anyone can verify this, or add any other geeky info, that would be great. It is a dually, by the way.
And, I also assume that this means no swapping of the innards. Bummer.
Thanks, all!
