I think all those coil overs would do is give you a harsher ride. You really need to find out why it sags. Is one side of the truck heavier than the other? Is the frame bent? Arc on the springs different from side to side? Assuming both your new shocks are identical, you will still have a sag (maybe less, but it will still be there).
My truck had the same problem and the PO put a coil over shock on the sagging side only to try and even it up. The bump stop on that side was bent from severe overload, so I know how the sag got there! I have new springs and rear end now.
If the springs need to be re-arced, maybe there's a shop near you who can do it cheaper than replacing the springs?
