Rear end not centered

I have the same problem on my 73 swinger . Tire to inside lip of quarter panel on drive side is 1" on passenger side it is 3/4" and tire sidewall rubs slightly . I was told that shackle bushing could use replacing or sometimes the pin on the bottom of the springs have a pin that fits into a hole on the spring perch , sometimes. They can shear , I was told to start there , sometimes just loosing u-bolts up taking a couple measurements and fighting the u-bolts back up could do the triick