My 98 LS400 had a slight ,to moderate vibration problem between 1500-2500 rpm's .Lexus dealer told me it sounds like a plugged fuel injector / injectors .But wanting to be certain before spending the $339.00 to have them cleaned ,I took it to a local mechanic by my home .He called my up after looking at it ,and it was my transmission mount .It was completly in two pieces .Put a new one in ,now it's smoothe as silk .Hope someone finds this helpful .

 

glad your problem was solved, the trans mount is a pretty common item to replace, and is ridiculously simple to do as long as you have a jack and some socket sets, should cost in total less than $40-50.