Hi Guys, newbie here. :)
Sorry to dig up an old topic, but my 91 Ls400 with 132k on the clock and running the standard 15" alloys has the same problem.
Above 80mph on the motorway the steering wheel starts vibrating like mad, you can also feel the vibrations through the floor and the seat. The vibrations i assume originating from the wheels or suspension. As a side note, over 100mph (which i know i shouldnt be doing ) and the vibrations seem to lessen. It ruins the otherwise enjoyable experience of the Lex, which with the exception of this problem loves a long motorway jaunt.