^^ I presume you meant H2, H4 n L4 in your question , not L2.
Anyway, H is high range, i.e the normal driving gear ratios etc. So 2 means its driving the rear drive shaft supplying power to two rear wheels and the 4 means its driving front and rear drive shafts driving all 4 wheels. The L4 is low ratio, various vehicles would have different low ratios between 1:1.8 going upto 1:2.8. In practicle terms you are roughly reducing the speed of your vehicle by 1/2, while doubling the power n torque to the wheels. So you would have lots more power to handle the difficult terrain, (where obviously you can't fast anyway) and becuse your speed is reduced you have more control over vehicle speed in that difficult terrain.
Vigo's would have part time transfer case, which means you have central transfer case locked (equivalent to centre diff lock in permenant 4 wheel drive vehicles) when you engage the 4X4 mode either H4 or L4.
This topic is originally about diff locks, which I presume was referring to individual rear and front diff locks and I don't think the Vigos have either, not even as an option.