I think they're all the same - even between open and LSD diffs. You'll need the small case ones, I'd suspect.
Mine are all really rusty too, but the seals hold & the bolts are tight so I let them be.
You can also swap the whole deal out...I swapped the OEM open small-case 4.10 diff for a medium case 3.73 LSD a while back. I do a lot of highway driving, I highly recommend the lower ratio.