I have a 1991 F250 7.3IDI 4x4 5 speed and regularly tow a tandem axle car hauler with my jeep cherokee on it. no problems.
Occasionally I tow the trailer/jeep and have a heavy old camper on the truck (scales at 15,800 lbs). no problems.
Mine has 3.55 gears and the hills really slow me down, but I can easily keep up with traffic on the flats (100-110kmh) and mileage is fairly decent.
I did some research before buying a tow rig and I wouldn't want to do this with an automatic equipped truck of that vintage.
I just finished a 2500km trip this past weekend (not towing) and my truck turned over 394,000 kms.
One other thing, I changed all the fluids to Royal Purple when I first got the truck - trans, tcase and diffs. I found in my jeep cherokee with the RP fluids that the temps came way down and that was good enough for me to use them in all my vehicles from now on.