95 E350 ride height

I have been trying to diagnose a handling problem on this van. It is a 15 passenger, I have put new tires on it and had it aligned twice, but it still wanders all over the road. Is it possible if the front springs get weak that they could be the cause of this problem? I notice that the front I-beams have hit the rubber bumpers on the frame quite often. Is this normal? How (and where) do you measure the ride height? The alignment shop could not tell me, leading me to believe that I should find a new shop. This is a passenger van, not a cargo van, would the moog cargo coils be a good choice? Information, as always, is appreciated!