Lightning Tech Question

OK, I'll try this one more time.
I have a friend at work with a 2003 Lightning. He has taken it into the shop three or four times because he says it is pulling to the right really hard. He is used to a vehicle pulling a little because of alignment or the bank of the roads here in Arizona but both dealerships he took it to can't find anything and they have even changed the tires twice. Do any of you know what may be causing the truck to do this. Any help will be really appreciated.

Thanks. He's just getting irritated with the whole thing because both dealerships can't seem to find anything wrong with it. We have rain grooves and a slight slant to the road because we live in the desert but he said he compensates for that with his driving style. But I will tell him abouth it being a combination of both.
Thanks once again.