2001 models began production on Jan 1, 2000. The 2001 model year was extended "back" (forward?) for Rams and PT Cruisers to meet 2001 CAFE requirements. The idea was that the PT, being a small fuel efficient "truck", would offset the low fuel economy of 1500 Rams. The ETH (HO diesel) engine was introduced with the 2001 models.
2001. 5 models began production sometime around *edit August of 2000*. The 2001. 5 is not a DC designation, it was created by Ram owners as a way to differentiate between the early versions of the 2001 model, and the later versions with disc brakes but fewer body moldings, grab handles, etc.
Dave