No offense to the previous poster... . DDP does ALL areas of work, maintenance, repair and performance. They do work on all makes of diesels too, not just Cummins. Lenny, Evan and Pete are all top notch wrenches.
Monroe is just a short drive from Lake Stevens, while Cummins NW is 45 miles away.
DDP is an outstanding shop, great service, knowledge and experience. Another cool thing, if you have them do any performance upgrades, they include a run on their in-house dyno!
Do not go to Dynamic in Snohomish, there was a lengthy thread on NW Bombers about their "service".