2001 vibration at highway speed

[knucklebump]

I've looked all over for a solution but no one seems to have the same symptoms. My 2001 base model 2wd 4runner shakes rather violently above 50mph. I took it to a tire shop and had the tires balanced, no difference. The vibration only happens with my foot on the gas. If I take it off and cruise, it's smooth sailing. Has anyone seen this before?

Thanks!