rear right tire thumping making the whole truck vibrate

[ase2009]

1985 chevy s10 2wd pick up 1.9l 119 2 bbl 81k miles

ok heres the story.... i had new tires put on the rear of my truck they are about 4 inches smaller in diameter then the old one and i used same rim and they were balanced..... so on my way home i accelerate to 40 mph and it starts to vibrate and kinda jump only above 40 mph it does this some times it will do it at 30 but not as noticeable please help me thank you.

[Shdwstrr]

I would take it back to the dealer and demand they check the balance.