I just took care of this problem by instalilng 'extra' stabilizer scissor jacks. When I'm done I will have 8 in total.. an additional 4 from the stock 4. With just 2 extra added, one on each side just forward of the wheels - I can tell the difference in firmness was night and day.
I posted about it this thread.
There are a lot of options - as I learned (nearly the hard way) make sure that whatever option you choose to go with doesn't void your warranty, if that's a concern.