Ya know, I had to basically take the report, hard code a parameter dropdownlist with a dataset, test it in VS, then take it off so the parameter would read from the URL.
Before I deployed the report, I had to delete it from the Report Server. I tried serveral different changes, deployed them, and they weren't overwritting the existing report on the server.
I knew the report would work since the original was working, it was just the "copies" of the report that were coming up with this error.
Thanks!