Does it sound like this?
If so, you've snapped your rear axle. You're looking at about a $1000 swap
Just kidding, it's a blown shock and it's really not all that uncommon. The fix is the cost of two new rear shocks, and about an hour of your time with some common shop tools. Do it yourself, it's easy, it's fun, and the most helpful WF member besides Jerry did a beautiful How To which you can find here!
make sure to PB Blast at least 15 mins in advance if not more
and feel free to ask me any questions you have.