diffs

so i got a good question thats probably been asked already but here it goes. ok so i bought a 1998 f-150 , 6" fabtech lift kit , 3" body lift , it already had the 37 x12.50 x 17 tires and eagle alloy rims. my question is this , me and my buddys were jus bs'n at the house yesterday and we got to talkin trucks , my buddy says shit , i think you need to get a new diff ,because your truck will end up blowin that stock diff to smithereens. i looked at him then looked at the back of the truck. the diff in my truck has a huge pumkin on it and looks nothing like the wifes diff in her truck. ive been doin alot of research on this and all i can find is that in 97-99 the f-150s came out with the 5.4 and a heavy duty diff ? anybody know where i might beable to find out more on my rig?