I have never done it on an automotive differential,
but on the bearing assemblies of large industrial pumps,
you measure the "breakaway torque"
with a conventional torque wrench,
but measure the "turning torque"
by wrapping several turns of strong fishing line around the shaft,
tie the loose end of the line to a hand held scale like a fishing scale,
then smoothly begin pulling the scale and line taut to get the shaft turning at a steady rpm.

Multiply the pounds of tension shown on the scale
by half the diameter of the pump shaft
and the result is the
inch-lbs of turning torque.

If a hand held scale is accurate,
and pulls against a wrench handle
at a right angle,
it is just as accurate as a torque wrench,
But this method
(And a conventional torque wrench)
Relies upon everything being stopped or steady
at the point of measurement,
because if there is any moving you have at least friction,
or worse, acceleration
which creates its own inertial force.