where to find 8.75 sure grip units used

Not sure about Mi, but the Pick-N-Pull's here in California seem to have a decent number of 70's Dodge pickups that they keep for a reasonable amount of time. They'd probably be the best bet percentage-wise to find a sure grip rear end in a stock vehicle. An entire 3rd member from there runs like $75. But they do go fast, I usually keep an eye on the inventory online and try to get down there if they get something in that's pre-'75.

Definitely not the bulk of their inventory, but it happens enough to be worth it to watch the inventories. I pulled an entire 8 3/4 rear end with the 11" brakes out of a '67 Polara a couple weeks back for $150. Not a sure grip or a good ratio, I think its a 2.76, but its a 742 chunk and big brakes, so it was well worth it.