This is gonna seem kinda stupid... BUT...(always a big but...)

Do the front diff and rear diff have to have the same ratios?

The reason why I ask this is because I am going to start a little project to get me and old beater 4x4 and put my engine and rearend on it. My rear end is a rockwell with 7.17 ratio. Common sense tells me that the ratios would need to be the same, but I don't know everything and would hate to screw something up for lack of knowledge...

 

Both differentials must be the same or one will get torn out.

 

That's kinda what I thought..Just checking.

Now I will need to find me a big nasty rockwell for the front as well..hahaha..

 

The front and rear diff's can have different ratios, as long as you make up the variance by using different tires front and rear. If you want the same tires front and rear, you need the same ratios.

 

I sure hope you don't spread that rumour any farther.
According to your theory one would have to be a a tractor size and the other a donut.