I'm trying to figure out what the gears are in my open diff Corp 14 bolt full floater. (There's no tag or markings on the axle that I can find)

With one wheel off the ground in neutral turning the driveshaft two full revolutions yields just under around 7 degrees less than 1 revolutions of the tire.

With both tires up the measurements were nonsensical as the wheels would take turns alternately moving or stopping as the driveshaft was turned by hand.

So with an open diff do you have to 'double' the drivehaft input to correctly determine gearing? by doing so it'd at least come close to the commonly heard of (by me anyway) ration of 4.xx:1 whereas I've never heard of something as numerically low as 2.xx:1.

Truck revs out on the highway but has a hard time hitting 100Km/h.

If you are spinning only one wheel you need to turn the tire two full revolutions and count how many revoutions of the drive shaft. Use a paint stick to mark the tire and the drive shaft to be more precise.

based off your measurements and what tony said about turning it twice if you only lift one wheel, 4.10 is likely. On another forum, someone stated that 4.10 was common on that 14 bolt diff. the math comes out to 4.07:1.