Unfortunately, a more expensive shock will (should anyway) give you a better ride. Those tires are not light, so the shock has it's work cut out keeping them in control. Just something to consider.
One more thing to consider, the stock shock mounts are pretty strong, but they are obviously only designed for 40lb tires and wheels. A 150-200 lb tire and wheel setup is far beyond their design capabilities. I used 39" tires and steel wheels with no problems, but I also had friends that ripped theirs off. Different shocks have differing amounts of control, my shocks where Trailmaster and where pretty soft. A hard shock will put more stress on the mounts.
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