How do you change the difflock fluid?

It is the same as any brakes.. you just have to use the levers instead of the pedal. With a speed bleeder you are either going to try and pressurise the reservoir which will not work unless you put clips on the pipe work; all the low pressure pipes are just push in so pressure would push them off.. or I guess you could push pressure/fluid back up from the slave bleed nipple. TBH it is very easy to bleed as there is a good fluid head (reservior high above slaves) and it pretty much all flows down hill.. so pulling levers is the easy option.

Agreed. I did both clutch and diff locks a couple of weeks back using Easibleed and it was a swift and trouble-free experience. But MB recommends 2 bar (30 psi) pressure, which is what I used...

You don't even need to operate the switches. The Eezibleed that I have got is pretty simple: one end to the spare tyre with 2 bar pressure, bottle 3/4 full of clean DOT4+ and other end screwed at the fluid reservoir that you have almost completely vacated of old fluid using a syringe. Just crawl undeneath your G and bleed the rear diff lock first and then the front one. No engine on, no switches, no bull...