4.10 gears in 7.5" Ranger axles are actually not all that uncommon in 1983-89 Rangers/Bronco2s
But one thing you will never (ever) find is factory 4.10's behind a 2.8/2.9 and a MANUAL transmission.
And yes, the "4" often hides under the head of the diff cover bolt that holds the tag to the axle.
BTW, if the Tag reads 4L10 plan on replacing (as a unit) the limited slip you find inside
if when you remove the cover you find several coil springs.
The Borg Warner limited slip unit is an absolute POS.
Though many were replaed under warranty after their abrubt and catastrophic failure
with "S-spring" ford built Traction-Lock units.
If OTOH your tag is reads: 4SL10 that means you have the later Ford built Traction Lock "S-spring" limited slip differential, which CAN be rebuilt (a clutch kit is ~$70)
AD