There really actually is a factory recommended way of testing your trac loc.
With one rear wheel still on the ground, raise the other side and remove the wheel.
Then you'll need "special tool 6790", which is nothing more than a steel plate with some holes for the wheel studs, and a "1/2 inch drive connector" on it. You could probably build something similiar by using the plate, drilling the holes for the wheel studs, and welding an old 1/2" drive socket to attach your torque wrench to.
Once you've attached the "special tool" to the wheel studs, spin it with your torque wrench. If the rotating torque is less that 30 ft/lbs on either wheel, then you need to rebuild it....
Ivan