Silverado Rear end ?'s

I have a 06 silverado std.cab, 4.3L V6, auto. It has a positrac rear end on it an i was wonderin how i can change that. An my buddy has has the same truck but its ex.cab with out posi an his kicks my ass, whats the deal? His truck should be heavier but he has better acceleration. Any help or sugestions would be appreciated. Thanx, Greg

what gears do u have and what gears does ur homie have?

their should be a code in the glovebox, something like GU6