For those of you with 98.5-2002 ISB engines, the oil pressure sending unit is a $180-250 piece from a parts house. Dealer price is unreal. Cummins offers a silly price on the part, which is worth about $20 tops in my opinion. I got Part number 4921511 for $85, which is a direct replacement.
There is a not so direct replacement, which is a single pin sender and wiring harness that adapts the single pin to the truck wiring. Part number is 3408428 from Cummins. You will need a reflash for the PCM so it understands the new sensor.
The sensor is located on the drivers side of the engine, below the ECM. It can be reached by taking a long extension through the wheel well, just behind the shock tower.