Figured this could go here since it's not specific (I think) to a 12 or 24 valve...

I'm not close to a Cummins dealer, either...

My tranny is out, so this is a great time to change the rear main seal... is it a pretty standard deal that the tranny shop could do, or does it require a bunch of Cummins super secret tools?

 

Easy job with no special tools. You can get the seal anywhere as long as the numbers on it are intact enough to read. Common job for any tranny shop.