stiffer rear springs

I wanted to know if there are stiffer rear springs available for my Jetta - my basic problem is this: I do a lot of trips in my car and load it up to the hilt. The thing is, the ass-end always sags and my mud guards scrape when I do hard cornering. Any suggestions for what kind of springs to look for - and can I only replace the rear or do I need to replace all four springs? Any help would be appreciated. Thanks.