Hi Bushbug,
Sorry, Roelf can't tell you what mods are allowed. He doesn't know and even if he did, he sure doesn't have the authority to do so

. I wouldn't take the word of anybody on the sales floor in that regard either. Remember a warranty claim is a very "cold" process that is handled in the corporate office, and they do not consider the opinion of the salesman who sold you the car.
Your other question:
1) Rear diff lock is very important when doing serious off-road stuff. I use my Pajero for holidays in the outback and the odd excursion into the mountains, I find the traction control more than adequate and have never bogged down in a Pajero. If I were to consider doing 4x4 trails and competing with the Jones', then I would like to have a rear diff lock.