I just bought an '05 4 Runner Limited and I'm experiencing rear end
movement. At about 60 mph on the freeway, the rear end starts to
wonder-fishtail. Has anyone else experienced this problem? I'm not sure
if it's the tires or the rear end. Any suggestions?

The rear end wanders? Maybe it just feels like that, due to bad shocks, or springs...?? I would ask Toyota to look under there for any abnormalities. It shouldn't feel that way.