Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
I'm getting ready to do one, can only tell you what I've heard here. I'm doing it to get rid of the rear blocks, apparently the flip results in a net 4" of lift, but since our trucks have saggy butts, I'm adding a zero rate for another inch. As far as difficulty, seems to vary. You have to remove your old springs and hanger, which involves getting the factory rivets off, heard it can be a pain. You also ideally replace all bushings in you springs with new greasables from ORD, aslo seems to be a pain to get the old ones out. When ready to mount the flip you have to either enlarge holes in frame or make new ones to mount it. There is a detailed write up in the tech section here and on offroaddesign.com, actually I just looked they are both here:http://offroaddesign.com/catalog/shacklekit.htm
A 4 inch spring is going to be a lot stiffer than a stock spring due largely in part to the shape of it (more arched). Rear blocks are nice because they are cheap and you retain a nice flexy rear spring. The problem is that you get a bit of axle wrap.
With a shackle flip you get the best of both worlds, a nice flexy spring without the axle wrap. If you are just going for a 4 inch lift and you already have the rear spring then you definitely won't need to do a shackle flip as well. On the other hand, you could do a 6-8 inch lift using that rear spring as well as a shackle flip.