Should I replace the Leaf Springs?

Here's the 4000-lb spring at SD Truck Springs. A number of us have used that vendor for various parts.

Please post your Axle Ratings. From your OP, 12000-lb, I'm taking rear at about 8000 and front at about 4000. Our 1983 E350 was rated a little less, 11000-something, but that's from memory. I didn't keep any of the old information.