At a steady 70mph cruise 3.73 will run about 2000-2100 rpm and 4.10 will be closer to the 2400-2600 rpm range. But to be sure you can jack up the rear end and make a mark on the drive shaft and the diff while lining up a mark near the bottom of your tire and the ground. Now spin the tire 10 complete revolutions while counting the # of complete revolutions the shaft makes. For a 4.10 the shaft should make 40 revolutions to the 10 reolutions of the tire, for a 3.73 it should be around 37-38 revolutions. I am pretty sure this is the correct way of doing it without opening up the diff.
Its pretty obvious you have an open diff. (no posi)

Perhaps a Nelson tune is what you are needing?