another thing if you have radial tires don not rotate them cris cross you can only rotate them from front to back on the same side or else the radial belt will become frayed within the rubber causing tire bounce and vibration also quick wear and eventually pop

you may also want to check out your transmission mount
if its soft or broke
or if the tranny bounces when pushed on that will also cause vibration sometimes not constent like it will come and go

Jeff