I agree with what Pope said, and add the following items that can contribute to your problem.

High tire air pressure on the front tires.
Very smooth road surface (as the polished cement pulling onto a gas station).
A short tow bar will not tow as well as a longer bar.
Not much caster (as previously mentioned).
Very little weight on the front tires, as on a VW Bug or a car wo/motor.
Steering box set with too much preload (loosen it up) or stiff tie rods etc.
To offset the locking differential effect during towing I would try to inflate the rear tires hard and let the front tire pressure down.

I've flat towed aircooled bugs thousands of miles and one of them had to have it's steering box loosened before it would track back after a turn.