Next on the docket for my 2005 E150 chateau wagon I think is rear leaf springs. They look fine and the wagon is level but it has the wallow and thump of bad springs and the new shocks made no difference to the ride. There's 153K miles on them and the previous owner did heavy trailering, and as springs seem cheap so might as well. MY QUESTION is what strength / weight capacity to replace with? I'm expecting to do some easy off road stuff and with some washboarded roads and I have heavy bed and 7 gallons of water etc in rear.