Differential seal and broken spring
 

I was installing a new diff with new seals and axles were getting stuck. After further inspection of the circlip, I found a piece of broken spring inside the diff. Is the seal shot given the spring is broken. I fished out the remainder of the spring using a magnet. Any advice, install as is or replace this seal. thanks

Replace seal, be more careful when installing axles next time.

I can't find any in stock seals at mazda local. Is the aftermarket seal ok to use ?

Only if you really enjoy replacing them. OEM is the only one that doesn't leak soon after install, much like cam and crank seals.