Been there , done that with my 86 D150. The problem started on the assembly line with a mixed bag of l/h lower control arms.

It seems that some 1 Ton control arms made their way into the 1/2 Ton control arms. The 1 Ton control arms have a deeper spring pocket, henceforth the noticeable sag on the lefthand side.

The fix is whats called a " correction rubber ", which is a thick piece of rubber to fit the top of the coil spring. Works like a charm. Don't know if it is still available.

There is/was a company making them out of aluminum, can't recall the name at the moment. Paul