Been there done that, my 2000 v6's rear struts blew a month after I got her. When I pushed down on the bumper the rear would bounce up and down 6-7 times. When the car is well, the rear should rise once and stop, 2 times at the very most. Are you sure you're rubbing your gas tank? I've bottomed out a couple times lightly and when I got under there the tank was fine, it was the stock exhaust (If I remember right) and I was told by Ford that it's not that damaging unless you hit it hard.
Consider replacing the struts with a more rigid system, it'll ride tighter and bottom out less, just a little bumpier.
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