If fluid level is correct and the fluid is still red and not brown/black then most likely you need a new transmission mount. When you change the mount the old one will probably fall apart, especially after 20+ years and 200k miles. The good news is that the part is cheap and you can change it yourself.
http://www.clublexus.com/forums/ls400/450071-diy-engine-and-transmission-bushings-replacement.html