Uhm.... the tow ratings on the bumper are what the bumper is rated for... not what you can tow...
BTW, this should be in the towing forum...
To answer the OP's question....
It varies based on your abilities, the gearing of the vehicle, manual or auto transmission, and size of motor.
Personally, I'd never hook anything to the bumper on a newer Ranger unless I had no other choice. I've seen stock bumpers twisted up from towing (plus I planned to exceed the tow rating of the bumper anyway), and I picked up a Class III hitch awhile back. With my 2wd, 3.0L Ranger with a 5-speed and 3.73 gears (plus I have the heavy spring package that upgraded my rear brakes to 10" drums), I have towed a Bronco II behind it several times both flat towing and on a tow dolly. I also towed a lifted YJ jeep wrangler behind it as well as a tow-behind compressor for a jackhammer.
I wouldn't recommend others trying to tow a BII on a tow dolly behind their Ranger unless they are used to towing, as it's enough weight that you can get in trouble if you're not careful.
So it comes down to the fact that we need more information on your vehicle specifics to be able to tell you the factory ratings. Plus it sort of depends on what your skill level is with towing as to what would be recommended. I believe a BII exceeds what my owners manual suggests for my Ranger as far as tow weights, but if I had an auto transmission it would be ok according to the book. I'm comfortable towing that weight and my truck seems to handle it fine so I do it anyway.