floor bounces under drivers seat/ not severe , but like an out of round tire

4x4 auto trans . At about 42 -47 m.p.h. the floor bounces in 2wd mode /not noticed in 4wd mode = started at around 60,000 miles . we have replaced tires and wheels / u joints / rear end lube with synthetic grease / lubed the drive shaft slip yoke with Ford lube / multiple tire balances / a couple of alignments / putting several hundred pounds in bed . Only acts up at 42-47 mph whether under load or coasting = up or down hill = in drive or neutral . Ford techs say they "have run out of ideas " and there are no service bulletins for the 4wd