I agree with what most nearly everyone has been saying here: with your vehicle on the ground, the shackle angle really isn't that bad. Again, while the "perfect shackle angle" is a debatable thing, you want your shackle to rest slightly aft of vertical in repose (rather than slightly fore of vertical, like it currently is). This will allow your springs plenty of flex and travel in the right direction.
Loading it with weight whilst it's static (NOT loading it and taking it wheeling) might be a good experiment too. It should create the necessary strain on the vehicle's springs to see how they behave. If they naturally flex aft rather than fore (in an inverted position), you may actually not require the longer shackles, at all. You can then try wheeling it, so as to get the springs to "settle". IF your springs flex towards an inverted position, I'd recommend you NOT wheel it whilst hoping for it to get better on its own. Worst case scenario: inverted shackles may shear away their mounts from the chassis. NOT the most desirable thing when you're on a trail.
IF the natural tendency of your springs whilst loaded is to invert the shackle, you can either a) get longer shackles b) flatten (and thus lengthen) the springs (which I wouldn't recommend, since it negates the concept of "lift springs") or c) reposition the shackle mount slightly fore of the current location. I'd probably go with a).
From the looks of the shackle angle on your unladen vehicle while it's parked on the ground, a longer shackle WILL address your problem, probably. Because it'll create more leverage, the flex in the springs will force the shackle backward, and the chances of inversion will be reduced. It'll also provide you with a little more lift, which, since you're looking to lift the vehicle anyways...can't really be a bad thing.
In any case, it'd be a wise thing to sort the suspension out before you seriously embark on any travels...because of the possibility of it souring any experiences you might have (and its potential to burn a large hole in your pocket, subsequently).
By the way, did you check to see if the spring stacks are off-centric (ie, comprised of unequal halves from the centre bolt outwards)?
Whoa. Holy Lengthy Monologue on my part!
