Thanks for your help all!
I had a few minutes of free time before work today and tried a quick fix to the problem. I had a spare piece of MDF about 40" long and 5" wide and duct-taped it to the back of the cabinet with a bit of some rope caulk in between. The part that's vibrating is the width of the tv starting at the top, sloping down about 2 1/2 feet.
Results - Hey it works great! It was not quite the maximum effect because I think one more board would do the trick, but the rattling of the chassis was greatly reduced. The 2 1/2 foot slope could not be completely dampend by the 5" wide MDF which was only a 1/2" thick.
Again, thanks for your responses. I knew someone else on here has had to have had the same problem with all the systems with big screen TV's and BASS that's present.
Oh yeah, the wife still hasn't noticed the KLF-20's. The speakers they replaced were somewhat similar in size, but no near the quality. I just thought I would see how long it takes her to notice.