differential moving after offset shackles.

[keithq69]

I have a 1969 Camaro that was mini tubbed when I bought it but was only running a 275 out back and the shackles were not offset.
I bought some new 19x12 wheels and 335 tires. I made my own offset shackles with 1/4" cold rolled flat bar.
When I first put the wheels on the car everything fit well, I had to cut the fender lip back a bit but the wheels sat well on each side.
I've put a few hundred miles on the car now and have noticed that the tire now sits closer to the drivers side by about 3/8" or so.
I'm going to investigate tonight to see what has aloud it to move.
Any thoughts on the most probable cause?
I'm also going to install a panhard bar this week just to make sure it doesn't hit on a hard corner. The tire fitment on the back is very tight.
Any thoughts on the panhard?