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+1 to all the above
The 2000 Rangers(1998-2000) get speed signal from rear axle ABS sensor, if you have 7.5" now and go to 8.8" then yes speedometer will be off a bit
The 3.73 with limited slip(L/S) would be an upgrade over a 4.10 OPEN
But as said, you will have a higher top speed but a slower time getting up to that top speed, that's the trade off
The 7.5" or 8.8" are of no benefit or drawback, its a neutral choice, whats available, and at a good price is the choice
1993 thru 2009 Ranger or Mazda B-series rear axles have the same width as your 2000 Ranger, so best choice
2010/11 rear axle were also same width and had rear disc brakes, but often cost alot more because of that
Rangers/Mazdas had 9" and 10" rear drum brake sizes, you just need to measure to see what the new axle has
Brake Backing plates for these can be hard to find so make sure new axle comes with them and they are undamaged, as these were often removed on wrecking yard axles because they are scarce