Well, the axle locates on a single peg on each leaf spring. Movement is really limited. Have you checked the wing width from the inner wing panel to the lip of the arch? There is often a few mm difference. A Panhard rod shouldn't be tensioned, but if you fitted one, then slackened the axle U bolts, in theory you could use the Panhard rod adjuster to help centralise the axle , before tightening everything. But also measure various points side to side. From the chassis.
