If you put a 2 inch lift kit on a D2,
do you need to change out the front and rear differentials to a 4.11 gear set?

Also, what are lockers, ARB air lockers?

 

You don't need to change your gearing because of a 2" lift. You might consider it if using tires over 32" diameter.
A locker locks the differential making it solid from one wheel to the other (or front axle to rear if talking about the Centre Diff), rather than having a differential between them. On our Disco's (which have a pretty good Traction Control), a locker is unnecessary until you're doing some pretty serious off roading.