I recommend that you get a Reunel rear bumper with hitch. Their bolt-on hitch (attached to their bumper) is rated 5000 lb. tongue, 15,000 lb. trailer; the welded Reunel hitch is rated 20,000 lb. Plus you will have a strong bumper instead of one made with 1/16" thick steel.
The Dodge factory hitch ratings are covered in your owner's manual. For example, the 2004 rear step bumper is rated Class III, for 5000 lb. trailers. The factory kunder bumper receiver hitch is rated "up to Class IV. " Class IV is 12,000 lb trailer, with 10% to 15% tongue weight.