Finding Front Gear Ratio
 

Hey, my brothers bronco that I’m now inheriting due to my own truck being stolen has a junkyard rear end in it that doesn’t match the front, and I’m making him buy me a new rear end that matches so 4x4 is usable (and so the truck actually has decent power lol). Problem is, we don’t know if the front gears have ever been re-geared, and he wants me to be sure of what they are before buying a rear end (door sticker says 3.55’s). I don’t see any sticker on the front diff, and I highly doubt anyone would re-hear them, but I’m at a loss on trying to manually find a way to do the ratio. I tried every combo of diff locked and unlocked, different tires on and off the ground, and I cant get the front wheel(s) to spin with the driveshaft. Is it even possible to calculate it that way? I’m not super knowledgeable on how all the 4x4 and the diffs work and stuff so any help in finding the gear ratio is greatly appreciated, thank you!

Does the truck have manual or auto front hubs?
With manual hubs simply turning the dial should lock it, with autos you may have to start by turning the front driveshaft to get them to lock.
If none of that works the hubs or diff are broken and you have more work to do... unfortunately.

The hubs lock manually, so if I have the front half of the truck up and it’s in 4x4 and the hubs are locked it should spin the front driveshaft?

You could pull the cover, count the pinion and ring gear teeth and divide.

Does your transfer case have a neutral?

IF you do, with both front wheels off the ground and hubs locked, you should be able make the driveshaft turn and count it out.

No you can't put it in 4wd that locks the front to the back which are still on the ground. Leave it in 2wd and you should be able to spin the front drivetrain.

Transfer case in 2wd
Lock passenger side hub (driver side unlocked)

Crawl under, put a mark on the driveshaft to keep track of rotations

Turn the driveshaft 3.5 rotations.

The driver side axle shaft should rotate TWICE.

The OP hasn’t told us what year truck or front axle. There is no cover to remove on a TTB axle.

You can if you have a 78 or 79. But if it's the 80+ with the TTB, it doesn't have a front cover. You have to completely disassemble the hubs and then pull the spindles and axles to be able to remove the pumpkin and examine the gears.
Even if it is a 78/79, that requires draining the fluid. Not a bad idea if it's never been done, but probably easier to just rotate the driveshaft and tires.

Partially. You want it in 2WD or neutral (preferrably 2WD). You want the front driveshaft to be able to spin freely. 4WD would lock the driveshaft and you couldn't spin anything.

I don't think there are any transfer cases on a full sized Bronco that don't have neutral. But still, it's safer to leave it in 2WD. The front shaft will still spin, but the back tires will be locked, which will keep it from potentially rolling.
Neutral would be useful if you were trying to count the rear diff.