Mark,
If you try running a panhard on your current leaf spring set up you end up just binding the suspension up.
This is because the panhard moves in an arc and will try to flex the leaf springs laterally.
The only way to get it working properly with leafs is to use ultra soft bushes, not recommended, or use something like this
Spring Bearing.
The spring bearing still causes some bind but nowhere near as much as hard bushes.
The Watts Linkage is better as far as if set up properly you will get zero lateral misalignment during suspension travel.
The other condieration is to find where your current roll centre is with the leaf springs and make sure that whatever method you try does not cause problems with this part of the geometry. Here's some more reading for you...
Leaf Spring TechIf you want to try something cheap and different have a go at setting the rear up as a z-link, similar to 4-bar but with 2 of the links running to the rear, you could probably use the existing leaf spring mounting points for this and just weld some brackets to the diff.