The cheapest way is to swap the ring and pinion in one or the other so they match. But, depending on what you are going to do with it and what axles you have now, you might want to do an axle swap instead.
BTW, if the front is "faster" than the rear (lower number) then its not as bad as than the other way, because the front tires tend to pull it along.
When they are slower, it sucks because they act like an anchor.
I know some mudders who have done it on purpose. A 3.89, say in the front, and a 4.10 in the rear.
Or, sometimes they just run slightly bigger tires in the front.
The early Jeeps, and I think some Toyotas did that from the factory, and Toyota may do it still.
But not that dramatically.
It would be like 4.11 in the rear, and 4.10 in the front.
