Rear spring

[dh81k30]

Hi folks,

Thought I would ask here first.

My '86 C10 swb has started driving a bit erratically lately. I am fairly certain that the front end is in good shape. It has only been about two months since it had lower ball joints installed and an alignment. The other day I had the opportunity to get it on a lift to look for loose parts. The only thing I found is that the right side of the rear axle appears to be sliding back and forth on the rear spring about 3/4 inch. I think that there is supposed to be a locating pin to prevent this sliding.

Does anyone know what I need to fix this problem? Can I fix it or will I need to take it to a spring shop?

Any help will be appreciated.