The thing is that these "ebay" style coilover sleeves have rediculous spring rates to start off with. The fronts are usually never a problem because thats where most of the weight is. The problem is that the back spring rates are wayyyy too high for the application. I'd made the mistake of installing one of these sleeve-style coilover kits (dropzone) a few years ago. The back end would hop around like mad. I had them replaced with Eibach sportlines on my stock shocks, two days later, which was still shitty, but at least they were a progressive spring so ride quality was close to stock.
Dude, the thing is that the spring rates on your coilovers are way too stiff for the shock and rebound properties of your stock shocks. This results in less-than-acceptable ride quality and performance. My advice would be to do one of the following:
1) Remove your coilovers alltogether and re-install your OEM equipment. Sell the coilovers and get yourself a set of progressive springs. If you can, buy some stiffer shocks. Tokico, Koni, and KYB are all affordable brands to go with for shocks, and Eibach, H&R, Tein, Neuspeed ect.. for springs. Or if you can afford it, go for a complete coilover setup.
2) Get the stiffest shock you can and replace your stock shocks with these. This would be a total hit or miss though. Who knows what the actual spring rate of some of these "one-size-fits-all" coilovers are.
And one more thing....a 3" drop is freeking stupid. It offers absolutely no performance increase, and without a proper camber kit, will wear out the inside track of your tires in a few months due to increased negative camber. Look for somewhere around the 1.25" - 1.75" area for a drop. This way, you'll have a ride that looks good, but also performs well in the twisties.
Any questions about a new suspention setup, post 'em up.