Hope this helps, but on my 99 GS300 I had the same problem at exactly 60 mph. Ok so I go to the dealer and have them rebalance. Same problem. Again rebalance. A little less. Got tired of the looking at the factory chrome 16's so I replaced with a VAPC 18 w/245 40 18 Nitto 555's. You'd think that something different would occur? Nah. At exactly 60 mph a slight vibration. It is so slight that I figure since it's mechanical, it's a car, I live w/it.
Ciao,
Jl