Want a cordless impact wrench. Have Ryobi batteries but they only sell an impact driver. Will my batteries work in a impact wrench manufactured by another company?

I'm sure the better part of them use the same batteries, and I know some of the casings and terminals on the battery packs actually interchange on a few. One example is Black & Decker/DeWalt with their 18v batteries.

One example is Black & Decker/DeWalt with their 18v batteries.
Thats because they are folded into the same company.

Other than the Dewalt/B&D cross over as mentioned above I doubt you will find any batteries that will mate to another brand. How much difference is there in between an impact wrench and an impact driver? In looking around I don't see a lot of companies even making a cordless impact wrench.

http://www.toolbarn.com/product/dewalt/DW059K-2/