Honestly, there's nothing wrong with the shackle standing straight up and down, especially with the stock springs. In fact, with any flat spring, you NEED the shackle to be pretty vertical or you can end up with a stiffer ride.
Also, on the GM's you have to be somewhat careful where you put the end of the spring so you don't hit the frame, so typically running a shackle at a kicked back angle involves a longer shackle.
When you step into the specialty aftermarket world, kicking the rear of the spring back can help out in some cases. One thing we typically do with a custom spring is stretch them an inch or two to help kick the shackle back.
As for the regular aftermarket world, keep in mind that most of these suspensions were designed in the 70's and 80's and they're not going to go back and re-do things when they're busy building IFS truck lifts now, so we're kind of stuck there. And most aftermarket companies built their springs around the existing shackle which limits what you can do with the spring.
This is where smaller companies like us that specialize in these trucks step in with custom springs, and where you can do some custom work yourself to build it just like you want it.
[start soapbox speech]
Something else to keep in mind: bolting a regular aftermarket spring into the truck like a Tuff Country EZride still works pretty good and is relatively inexpensive which leaves you time and money to work on other important offroad mods like the drivetrain, body armor, recovery gear, etc. Avoid spending too much in one place like the suspension to the detriment of other important stuff. A better shackle angle on the front will not winch you out of a stuck.
When you have the rest of the goodies to the point your happy, that's the time to start optimizing each system more thoroughly.
[soapbox off]