Hi Dacey, there are lots of great tracks in CO. Also check out http://www.thecoloradokarter.com for more localized information. As David mentioned, the LO206 racing is affordable and great fun with huge fields. Set up an account and introduce yourself on the colorado karter site. You’ll find a great group of knowledgeable (and funny) karters. Pick up a used chassis and a Briggs and come on out.
As to your DD2 question, there is an active Rotax group that races at centennial but I think DD2 is in the single digits as far as participation.
We’d love to have another shifter guy to race with but if you are just getting back into racing, I would point you towards the briggs for affordability and simplicity.