There are 3 basic types of differentials used on Land Rovers (the RR 4.6HSE is not included in this descriotion as it has alot of unique compnents that will confuse the issue)

Rover Diff

What's it fitted to?
Front and rear on: All Series 1's, all Series 2's, all Series 3 88", all 90's, all Range Rovers, all Discoveries
It is also fitted to the front axle of all Series 3 109", Series 3s 109, 110s, 130s

How do I spot it ?
The rover diff is the very first ever used, in the first Series ones in 1948.It can be recognised as the smallest of the Land Rover diffs, it is completely removable as a unit from the axle housing. The housing has a downwards bulge in it to make space for the corwneheel inside.

What's the difference ?
All the casings are mechanically interchangable but not recommended.
The insides are different between 4 main era's. 
Start (1948) to before first Range Rover (1970) : 2 pinion gears 10 spline and 4.71 ratio
First Range Rover to First 3.9EFi Range Rover (1987?) : 2 pinion gears 10 spline and 3.54 Ratio
First EFI Range Rover to Disco I : 4 pinion gears 24 spline and 3.54 ratio
Later Disco 1 onwards : 4 pinion gears, 24 spline, 3.54 ratio with multispline main pinion. (Rubber propshaft coupling

Ratios:
4.7:1 All leaf sprung vehicles
3.54:1 All coil sprung vehicles
There are other ratios like 3.8:1 and 4.1:1 and can be found on some
rover cars.

This diff must not be confused with the so called "Wolf" diff which is
also completely removable from the axle but it has a much shorter nose
(pinion shaft) and is stronger than all the rover versions. It is fitted
to some late model Defender 110 and 130 models.
It is the same as the Discovery II diffs, which have one in the front
and one at the rear.
You can get ARB lockers for these too !

Ratios: 3.54:1 only
 

Salisbury Diff

What's it fitted to?
Front and rear on: All Series 2B Forward Controls, Series 3 "1Ton" models, Forward Control 101, Some heavy duty 110s.
It is also fitted to the rear of all Series 3 109" rear, 110 and 130 Rear

How do I spot it?
The Salisbury diff is much larger than the Rover unit

You can also get ARB lockers for these

Ratios:
5.7:1 (I think) in the Forward control 101 only (With HUGE sideshafts) 4.7:1 in the Series vehicles 3.54:1 in all coil sprung vehicles
 

ENV Diff

ENV diffs were used only on Forward Control 2A and early 2B - these
diffs are also fully removable like the Rover models but they are
massively strong with huge side shafts.
They are available in standard Series land rover width and there is also
a wide version as fitted to the early Series 2B Forward Controls.

You can't get lockers for them :-(

The Millbug uses these axles in the narrow form.