Look on the door tag to see what you have now, Read here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes
All Rangers from '83 to '89 came with 7.5" axles, so your '87 will be 7.5"
Because you have a 4x4 both front and rear differentials need to have the same ratio.
And you may want to change that ratio if you get even larger diameter tires.
4.10 ratio has better low end power(common on trucks with "tow package")
3.73 ratio is the middle of the power band ratio(most common ratio on Rangers)
3.45/3.55 ratio gets better MPG at highway speeds but loses low end power(found mostly on 4 cylinder Ranger, for better MPG)
Larger diameter tires LOWERS the axle ratio, so if you have 3.73 ratio 31" tire would make it "drive like" 3.30 ratio so loss of low end acceleration, 35" would make it drive even lower so low end power would be almost gone, even with V8.
Read here about the 7.5": Ford Ranger/Bronco II 7.5-Inch Axle
Read here on the front axles: The Ford Ranger Dana 28 & Dana 35 Front 4x4 Axle
You should have a Dana 28 on a '87
The 8.8" axles were used on the 4.0l Rangers starting in 1990, and should be a direct fit into an '87 from any Ranger up to 1997
Good read here on those: http://www.therangerstation.com/tech...8_8-axle.shtml
An OPEN differential means that the easiest wheel to turn gets all the power, (which is why only 1 wheel spins if you get struck), this is the most common type on cars and trucks, Ranger 4x4's often had L/S(limited slip) in the rear differential but not always.
L/S or Trac Lok(Ford's name, Chevy calls theirs "posi-traction"), has clutch plates in the differential, if one wheel starts to spin faster than the other the clutch plates will transfer some of that spin power to the other wheel, so both wheels get power in slippery conditions.
These require a special additive in the oil, because when you drive around a corner the outside wheel spins faster than the inside wheel, this would cause the trac-lok to start engaging, the additive allows slight slipping on the clutch plates