I break the diff in a little differently, in case it breaks in and starts to slip (slipping is ALWAYS bad).
1) Hold the truck up off the ground or table by the motorplate, with all four wheels off the surface
2) Put one rear tire on the surface, and apply a little bit of throttle and let it spin the lifted tire for about 10 seconds
3) Repeat for the opposite side, and then over again, about 5 times.
To check the diff while it's in the truck (just like the allen wrench test, but while it's in the car):
1) Tighten the slipper all the way down
2) Hold the right rear tire and the spur gear with your right hand
3) Try to turn the left tire...again, it shouldn't move!
To check your slipper, adjust your slipper where you like it, and then perform the same test again. The nut and spring will turn if done correctly.
This is basically what stu said, but it's easier on your hands