So I spent no shit like 2 hours looking for these things last weekend. Total unobtanium. I welded my broken washer back together, ground it flat again, and am just missing the 4 or 5 teeth on the melted part. No idea how it will hold up over time.
I've considered trying to take generic internally-splined lock washers
[like so] and seeing if any of them will jam on there and do the job. I've also considered giving my non-broken washer to some sort of laser/waterjet metal-cutting place and saying "make 50 of these", but I have no idea how a place like that works, how to find a good one, or if machine time for something like that would be like a thousand bucks and not worth it.
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Just for reference, the Ford part # for the multiple pieces (washer, spacer, splined washer) on my axle (I believe a '93) is F57Z-3B458-A.
There's also another part that basically combines the 3 pieces into one thicker splined washer, FOTZ-3B458-A. These were actually marked in-stock a lot of places until I tried to buy some a few months ago, at which point they flipped to accurately showing No Longer Available.
It's hypothetically possible that either of these part numbers may pop up on, say, ebay at some point, but I've never seen them.
DIscussion
[here] turns up the aftermarket part number WRN37891, but that was 10 years ago. Warn was sold to Randy's Ring & Pinion, and I read in another discussion that someone tried contacting them about the splined washers, and was told that part is now NLA as well.
Every other discussion I've read through ends up with people suggesting "go to the junkyard", which is obviously less than helpful if you don't live near a good one, and also less and less helpful over time. Unfortunately, the number of people still trying to keep this axle in service after 30 years is pretty negligible.