Miatas are made to be driven hard, right?
When you're abusing the car on the race track, the weak points become known.
One of these is the OEM diff housing.
Mazda made the OEM diff housing with a notch in the passenger side so that the diff housing will break in the event of an impact, for safety reasons.

In a racing environment, this is not ideal.
Broken diff housings are common when bump drafting in Spec Miata racing.
High horsepower Miata and drift cars are also prone to breaking the OEM diff housings.
The SOLUTION?
A weld reinforced differential housing from Mr Miata Parts using the Spec Miata racing approved diff reinforcement plate set.

You will receive one 1.8L diff housing with the reinforcement plates welded on.
See photos. As shown. (Typical photos)
Fits 1994-2005 Miata. 94 95 96 97 99 00 2000 01 02 03 04 05 NA NB 1.8L 1.8

Will come with the used OEM diff bushings installed in the housing.
The housing will also have a pair of used axle seals installed.

You could install this assembly in your Miata, as is.
-OR-
It is Recommended to replace the axle seals with a fresh seal set.
(You can choose the drop down option to add a new set of axle seals)

zoom-zoom.