I’d get a power washer in there and make sure they were totally free of any crud. Then I’d let them dry out thoroughly (a good road trip in warm weather will do the trick) and then I’d oil them with WD-40. I get my truck oil sprayed (oil guard) with the original formula -the one that drips for a few days afterwards. I always ask them to make sure the leaf springs get a good application and this has pretty much solved any squeaks. They also sell the oil by the can so you can reapply it if and when the problem recurs.