Hi Dan,
Are you sure the curb weight of your Colorado is over 5000 pounds? Double check that one. If under 5k, as long as you don't fill it with supplies and other stuff beyond 5000, you'll be in great shape, just as you are.
We tow a 4200 pound Jeep Liberty with our 2007 2350 on a 2007 E350 with V10 engine. Simply said, absolutely no issues towing it. We do have a decent tow brake which helps to stop the train better than if not towing at all.
The only issue we have towing is that the front tires of the Liberty take a beating of uneven tread wear on the inside edges, from sharp cornering "left then right" or "right then left" in parking lots and gas stations. Our Liberty has stock sized tires & wheels. I would imagine it would be much worse if changing the tires and wheels to something bigger, wider, wider stance, more off-road aggressive.
Ron