Somethin's funky with that rear suspension on your truck. It should NOT do that!
One of my 400's is dropped 2/4. Shackles & hangers. Handles fine. With the rear sway bar, better than stock. Still gets used as a truck.
Good shocks help. I have a bin with all the ones I've tried. Can't recommend the Belltech Street Performers enough! Love 'em!
Urethane bushings help, too. A complete kit is fairly cheap. Less than $150.00. But, the control arm bushings can be somewhat of a pitA to install. I picked up a set of used upper & lower control arms (wreckers) cleaned 'em, painted 'em, installed new ball-joints, & had the bushings installed (needed to do that in a press) so that when it came to do the lowering, the truck spent less down-time on a hoist, which costs me. Straight up Re-&-Re of the arms is fairly quick & easy.
While that is all apart, check to see if the upper control arm mounts have had the knock-outs for the camber adjustment punched. That may be why you can't get a proper alignment. A good alignment shop should know about that & have the right tool to do the job. If that baffles them, move on!
Like someotherguy said. Check that spring pack. If they've been really messed with, stock ones @ the wreckers are super common, & that equals cheap!
Don't waste your time & money on lowering leafs.
And I'll say it, too. Make sure that that is the height that you're gonna stick with. Those hangers are a pitA!!! Lowering is a disease, & a little is never enough!
How is the front lowered now? With springs or with spindles? If with spindles, that uses a stock length shock.
Best to decide now if you want to go lower, as with a 4/6, you will need the 2" shorter shocks
Once you get that rear suspension figured, make sure to check the pinion angle after the right parts are all installed.
To do that, the spring center bolt head may have to be extended in order to work with whatever pinion wedge is required. I had to do that to mine even just for a 1.5* wedge.
So, I've learned my lesson!
I will NEVER do another 2/4 drop. Ever!
For one, it's not enough!
Two, the 4/7 drop on my black truck was a whole lot less hassle, both with the install of the parts & the follow-up dial-in.
Three, IT"S NOT ENOUGH!!! Really! It just isn't!