Another down side to the torsion bars cranking is it will destroy CV joints easier because they're at a steeper angle and it will void your warrenty if you do it too much.
NO AFTERMARKET PART WILL EVER VOID YOUR WARRENTY UNLESS IT DIRECTLY CAUSES WHATEVER THE PROBLEM IS. So, if you put an aftermarket oil pan on your engine and it leaks and your motor throws a rod, obviously they won't warrenty it. It's the law that they must prove that the aftermarket pan had a hole in it or otherwise caused it though.
If it's autotrac it will need a new front driveshaft most likely, that's what all the books say. My buddies that have lifted 88-98s have the 6" Pro-Comp and love everything except that they had to pay $3000 and leave their truck somewhere for two weeks to get it installed.
If I had IFS I wouldn't waste my money on a 4". Just go 6". It's about the same price.
If your dad doesn't like the $3k price with installation, it's body lift time!
Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles