I have an '03 7.3 CC 4wd with 35/12.50's on it. I have the front leveled by way of an extra leaf spring from the local spring shop. I like the way my truck looks right now but my tires scrub my fenders (not the liners but the rear corners of the fender flares) under very little compression. I was curious what the pros and cons were to each.
my understanding of it is that shackles will change your caster and depending on how big they are can effect your pinion angle, i went with spring packs, theyre all over ebay for around 100 bucks with u bolts. ive heard a few horror stories of people shackling up the front and the drive shaft slides apart at the slip yoke leaving very little spline contact area and locking it in and stripping out the driveshaft... just my .02
running a mini pack on my current truck, trouble-free almost two years now. ran a lift shackle on my last super duty, along with 4" springs and had no ill-effects either. the caster/pinion angle concerns makes perfect sense in theory, but i literally couldn't tell any difference (and i wheeled that truck, wasn't a street queen).
It's hard to go wrong with springs packs. Shackles can open up a whole new can of worms like Airframes4life stated, and can end up costing more in the long run. Since shackle lift only give you half amount of lift, you are going to need a lot of shackle to get any decent lift. I think anymore than 2'' (1'' lift) of shackle is beginning to pushing it.
I would just use lift blocks and be done with it.
Edit: The "lift blocks" was pure sarcasm. It is the absolute wrong way to do it.
before i put my new pro comp 2.5" springs on i had the stock 4400lb front springs+tuff country 1.5" add a leaf+ 1" pro form lift shackle for almost four years and i never had any handling issues, other than a dump truck ride from the add a leaf. I changed the springs because i couldn't take the ride anymore and im debating adding my 1" lift shackle again to get me a little extra height up front
Will spring packs effect the ride or handling? Another question I've had since researching this topic is in regards to lift blocks. If the main concern is them getting spit out, why would a bolt through the center like the spring packs not work?