You'd be a lot smarter going with a 4" suspension lift if you are planning to add a 3" body lift to it. A LOT STRONGER setup than using a 6" suspension lift. The CV angles stay a lot closer to factory spec. Running the CVs at extreme angles are a sure way to watch them go snap!!!
4" of suspenion with 3" of body gives you plenty of room for full articulation with 37s.

This be what you get when you run 37's on 6" of suspension lift:

 

The fabtech and procomp kits leave you with cv angles better than stock because they include differential drop brackets. Does the 4" rancho kit even touch the diff?

A body lift will have no effect on backspacing one way or another.

 

Body lift will have no effect on backspacing. However, you will have additional clearance over the tires. 6" should be fine if you drive it only on the road and maybe minor off roading. If mostly on road, I'd say go 6" plus body. Or you could raffle your stuff off here!!!

 

It's my understanding that Pro Comp added the diff drop brackets for the '06+ kit due to CV failures. Is entirely possible that Fabtech has followed suite and includes drop brackets as well, but last I knew, the '02-'05 kits did not. However, I've not touched one in about 3 years, so it's possible. I'll have to look into that.

The Rancho kit retains the stock CV angle...

 

The fabtech has diff drop brackets for the 02-05 kit as well. Sherdep just installed one.

How can the rancho provide 4" of lift and still retain the stock cv angles without dropping the diff?

I'm not trying to bust yer stones. I'm just trying to make sure we get accurate info out there.