I have a used unknown mileage MSM 4.1 rear diff, just the pumpkin. It appears to be in good condition, rotates freely, none of the gear surfaces are damaged at all.
Asking $700 + shipping. Located in Northeastern PA, 18360, open to lower offers.
This was given to me as part of an insurance covered repair when a pot hole hit snapped my original housing arm. I insisted that they not replace the whole diff because I had swapped to a 3.63 R+P already, and I didn't want them to just toss that and re-install a 4.1. They ordered a full MSM diff for some reason instead of just a housing, separated the housing and swapped it onto my pumpkin. They were just going to toss the pumpkin in the trash and offered it to me for free. (I 'paid' plenty between deductible and rental etc... this isn't just milking a free item)
In order to put this in a non-MSM, you will need a diff housing with bushings. Any diff housing from a 1.8L will fit. 1.6L diff housings will not. You could keep your housing and swap on this pumpkin, similar to my repair, or source another housing for about $100
You need MSM specific axles. These are the larger axles that were unique to the MSM, non-MSM axles do not fit into this diff as they are too small. on the diff end. MSM axles WILL fit all NA/NB Miata rear hubs, they are the same on the hub end. MSM axles may be extremely difficult to find.
I have been assuming that I won't find anyone interested since this isn't a 'complete set', but posting just in case someone is. Worst case, it is a 'spare' for me, although MSM diff failure is apparently absurdly rare so I will likely never need it as a spare.
Also, if you aren't interested as-is, but would be interested if I obtained a housing and set of MSM axles, let me know that and what kind of price you would be willing to pay if I can get them to make it a complete set.