tire ballooning in the rear, but not front...

well i ran it again and bent a rear cvd.
luckily, i had a replacement. while i had the wheel off, i went ahead and rotated the wheels, front to back.
put it back together and the behavior follows the wheels!
there are two out of the four wheels that balloon like crazy and wobble bad.
i built all four of them at the same time with proline low profile 40 series inserts. they have not gotten wet.
why would they all of a sudden do this?
anything i can do?
maybe they are full of sand. i did run in sand the other day that my front diff blew. maybe since only the rear were spinning, they got full of sand through the air vent hole.
how can i get the sand out without taking the tires off the wheels? they are glued.