Ring & Pinion Gears
 

Are the S1 and S2 diffs and R&P's the same dimensions?

i.e. could I swap a LSD from a S1 pumpkin into an S2? OR could I swap a R&P gear set from an S1 into an S2?

yes, or you can just swap the entire differential assembly

I'm just interested in the 5.125 R&P set. On Mazda Motorsports' web site, it references only 04-05 RX-8's.

it will fit either one, it needs to be the limited slip model, some of the automatic cars don't have limited slip differentials