Why is my ride so harsh with coils?

I bought a 99 4.0 and it had been converted to coil springs and it rides horribly. I am not sure what brand of conversion it is. Any ideas as to why or what I can do? I have never had a vehicle that rides so bad. It looks like the suspension bushings are newish as well as the shocks. The vehicle was really well taken care of maintenance wise by the PO. Is it just the springs? Does anyone make a cushy spring suspension for these? I currently have newer 19" factory wheels with the factory Michelin 4x4 Synchrone tires.