Those springs as you describe are just to stiff for your app. The rear spring eye should be behind top eye mount not in front.

The fix, since you have the springs already/maybe is to have them dearched abit. either that or get different springs. Longer shackels arent the fix, as you found out. mike

Edit, Another fix as Cab stated is to move the upper spring mount forward, and that might be your easiest fix. Since you already have the springs.