To one Old Goat (from another),
I've got a '41 GMC that did the same thing. My observation is that the driver's side has the steering box located above the springs and the seeping, leaking oil/grease from the steering box lubricates the spring leaves better than the passenger side. The result is that the spring sees more of a load because of less friction and tends to sag over time.
My fix is to re-arch the springs. I do my own arching, but a truck spring shop can do it very cheaply. You might even be able to switch leaves between left and right springs...in other words, make your spring stacks a combination of every other leaf from the opposite side.
That wouldn't cost you anything and you could clean your springs up real good in the process.
Just my two cents worth.
Jim
