Swapping rear differential gear ratio sizes...

[jfalcon]

So, I'm dealing with this rear differential thing. I have a 4.8i that RealOEM says should be a 3.91 gear ratio according to the VIN. I couldn't find any images of one that have show the same markings as the one I pulled off my vehicle.

In fact, the numbers off the one I pulled show for a 3.46 gear ratio according to the numbers on the casing - I'm not sure if that is normal since I cannot find images of a 3.91:1 differential - and the one I was wrongly sold was a 3.15 gear ratio for a F15.

So obviously the F15 one isn't going to work (it's for sale if anyone is interested.. )

There is a 3.0 being parted out near me that has a rear differential out and ready to go. Question is: What would be the risks and gotchas here? Ultimately, I just need to get the vehicle back on the road for a while as I plan to switch to a hybrid when I can, but need a little advice on what way to move forward with this and spending even more money.

Thanks!