87 is an OPEN 7.5" with 4.10 ratio
Are you SURE it is 87?
Because ALL Ford Rangers from 1990 to 2011 with 4.0L engine got 8.8" rear axles
The word ALL from Ford probably only means "most", lol
4.10 gives good low end power for loads and trailers, also better for running taller tires.
OPEN means only 1 rear wheel will get power, which is odd, 4x4s almost always came with L/S, limited slip, rear axle, front axle is OPEN.
3.73 ratio is what most Rangers got, less pulling power but lower RPM at highway speeds so slightly better MPG, but really depends on driver .
You can get any 7.5" ring and pinion set to swap in, BUT(big but) you have to do BOTH front and rear, because of the 4x4 part, can't just do the rear unless you drop(remove) the front drive shaft.
List of the AXLE codes here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
For when you are at the junk yard