Welcome along
A 1999 should be fitted with auto hubs that engage when the transfer is put into 4wd.
Being tail happy is a sign that the rear diff oil is ready for a change.Needs to be LSD oil.
After market switches are anybodies guess to be honest but a bit of wiring tracing might reveal what they are supposed to do.
The rocker switch for the auto transmission has 3 settings.S is sport mode which holds the gears slightly longer,centre position,AT is normal driving mode and the snow flake will use a higher gear to pull off to avoid wheelspin.
Looking at your picture i can see the bonnet scoop for the intercooler is dead centre so will be the 2.7tdi motor.