Well seeing as you have a decent set of shocks I'd start with the simple, cheap and easy stuff first.

1) Check to see if you have 1.5" of clearance between the pinion snubber and the body. If it's too close the car will tend to porpous.

2) Spring clamps are your friend Clamp every leaf in the front segment to the main leaf. This makes the front of the leaf spring stiffer.
This is the Mopar traction bar

3) If the above doesn't work try removing the pinion snubber and see what happens

4) If all the above fails check into some new shocks. Some adjustable Rancho's or QA-1 Stocker Stars.

If 1 & 2 don't get you squared away I'd be suprised