Cummins P/N
If it is the diaphram canister type vacuum pump (two pods) you can get the remanufactured canister cheaper than the $300 plus from Chrysler or Cummins. Cardone P/N 641300 from just about any parts store, Advanced Auto Parts, Checker etc. My local parts warehouse had it in stock for $167. 00.
The Cummins vacuum pump seal kit P/N 4089742, gasket, seals and o-ring I believe. Under $10. 00
You can get them separate too,
Gasket P/N 3938655
Seal O-rings P/N 3910260 - Qty-2
Seal O-ring P/N 3911805
I would check the vacuum from both pods separately first, might get lucky and only be a bad o-ring seal.