i have a 2000 forester with a bad transmission the one that came out is tz1a3zc2aa-p8 the replacement i have is from an 01 outback would i need to swap the diff as well?

Not much help, but if you have the rear diff out, there is a code painted on the top of the housing.

Also, If the OB is 2.5 and not the H6 3.0, they should both be 4.44 FDR

I use the http://opposedforces.com/parts/ website, choose the model>year range>specific model info. Choose the 'Train' category and then the Diff (transmission) to view the 'G/R' / gear ratio I'm guessing.  

 

4-cylinder Automatic Foresters and Outbacks in that vintage should both be 4.44.

Best way is to use the VINs from both vehicles on www.partsouq.com. If you look up the ring/pinion part number for the front or rear, the gear ratio will be in the comments.