Well, I guess we are starting off with the ratio of the speedo/speedo cable
to the wheels and really the ratio of the speedo/cable to the driveshaft.
All I know to do is change to different colored gear on the tranny output
shaft although possibly the spiral gear ON the shaft may be changeable too.
(Definitely not as easy though!)
The accuracy there can be judged by the behavior of the odometer. If it is on,
you have the right gears. If not..... (Now there may also be different ratio
speedometers - I've seen them marked 0,99 and 1,03, for example - crazy
Europeans don't know a comma from a decimal point) Anyway that would make
a 4% error where you have more like a 20% error. If you have different
speedos you can swap that might be fun to try. That and the gears are all I
know for that.
Now once the odo is correct you can start in on the speedo. Dirt in the
coupling will make it fast and jerky, and dirt in the bearings will make it
slow. Tightening the spring will make it faster and loosening will make it
slower. Best starting point is to start with it clean and lubed with the
lightest machine oil you can find - NOT any kind of penetrating oil!!
If you can find specs of rpm vs mph and have a way to drive it at a constant
speed that would make it easy, just like a speedometer shop. Do we ever do
anything the easy way?? Probably rarely at best. (Sorry about the pessimism
but ¿Whó started this, anyway?)

About Spanish for limited slip - dunno. 4WD is «doble tracción». Maybe
something like double engagement or firm engagement - perhaps «diferencial
de embrague firme» or some such. Terminology varies from country to country
also. Or maybe you could go from what it does - «diferencial que no pierde
tracción», differential that doesn't lose traction. How about «Positracción»??
(I can manufacture words in Espanglés in a hartbeet!)

Good luck - I think you're right.
--
George Downs Bartlesville, Heart of the USA!