You do indeed need to divide it by diff ratio, but also the ratio of the gear you are in when the dyno run is done.
Example: 3.08:1 diff and dyno run done in second gear, which might be 1.63:1. Lets say the dyno reads 420NM torque at the wheels after those reductions, so divide that by the diff ratio first, which gives you 136.36NM at the tailshaft. Divide that by the 2nd gear ratio and you get 83.66NM at the flywheel.
Note that this torque reading is still lower than the real torque figure because the engine still has to drive the power steering pump, water pump, alternator, etc.
Also note that I am 100% NOT CONFIDENT in my analysis there, but that is how I understand it would work.
I would be thrilled if someone could either confirm or deny that what I have said is accurate, made up dyno'd torque figure aside.