Shock extender is not for correcting a ride height issue. It's to help correct the angle the shock operates at so it can be more effective at its job...which is dampening wheel bounce.
That rear is sagging and it's probably because it either has drop shackles already installed (OEM shackles are very short and look nothing like fabbed drop shackles so it should be obvious if you look) or the springs have been seriously overloaded, or torched, or had leafs removed.
On fronts don't do spacers, you would need more thickness than is safe for a slip-on spacer (not enough stud length) to get the clearance you need, just carefully trim the control arms as mentioned. It's been documented a bunch so you should be able to find pics with some searching. Or as also said, go up in wheel size.
Richard
