I've had most of my work done here:
In Littleton - west side C470 and Ken Caryl, Excel Diesel. Ralph is the Store manager.
They are a bit pricey, but usually able to haggle some % down depending on what's being done.
In Denver - Diesels only. Just west of I-25, on Alameda. Smaller shop, but been around for a long time.
In Castle Rock - Douglas County Diesel.
In Co. Springs, - would recommend Patterson Custom Diesel or Dirt Road Diesel.
What are you needing to have done? I may have one more shop I can recommend depending....