Hi there. As far as I know, the upper light cluster has the brake lights. But you can take out the bulb to check if it's a twin element bulb.
As for the diff,
a) it should be in high range, diff UN-locked.
b) off road, low range most of the time (unless it's level ground), and the diff depends on the surface. If there's a chance of spinning the wheels and loosing grip, lock the diff. But remember to un-lock it as soon as you get onto solid ground or you'll break things.
c) hi or low, depending on how fast you want to go. If traction is a problem, lock the diff, but remember to un-lock it as soon as you get back onto tarmac.
Hope this helps.
