FRONT differential and Transfer case fluids...how often do you change yours?

[Qxxx4]

I guess every 2 years is the rule of thumb but do you guys let it go longer? how important is this if i rarely use 4x4....and is my transfer case being used evn in 2 wheel drive? i already changed my rear diff fluid it was over due, the other 2 appear ok but its been 2 years...guessing the fluid isnt as stable as it should be and is it that important to change, thanks

[Empty V]

Bump.