There is a 255/80/17, but it's a BFG KM2, which is a MT, and the decrease in rpm's will probably be offset by the added rolling resistance of a MT. It is a little wider, but still fits the DRW without clearance issues.
HPCR's are less effected by rpms than older mechanical diesels.
On a side note, all motors in 2006 were HO and produce 325hp, and 610 ft/lbs.