ok here's what i have done in the past
1 ful tank transfer switch from 1985 chevy p/u with dual tanks
1 switch of what ever you chose
12 volt power to the switch
switch turns on and off the transfer swtich
thus you now have 4wd as an option by fliping the switch
i ran this 4 years in my yj with a lock right in the front on hard roack it was at times hard to turn so off with the switch 1 wheel drive turn and back in
also on a yj the yellow hose is 4wd and green will get you 2wd
you need constant vac to pefrom the act so if you want 2wd vacume to the green hose dose it