That should be a Eaton model 1350 vacuum two speed rear axle. That was standard for a 350 series GMC. It would be nice if you could find a Eaton model 16600 from a 450 series GMC COE, or Eaton model 17501 from a 470 series GMC COE. They are both electric shift two speed axles. No need for all that plumbing.
