If you've taken that rig muddin', then it is entirely possible that you spun the rubber on the rim. If you go from 0 RPM to lead foot RPM in mud, that puts a lot of load on the entire system including the beads, thereby creating a potential for spinning the tire on the rim.
With regard to the the huge weight, it is sometimes possible for a shop to spin the tire on the rim until it is balanced as best as possible before adding weight. This makes sense since the rims themselves are not perfect, so putting the high sections on the rim and tire opposite eachother provides some degree of offset. This way, the weights that are put on are minimal.
This however, takes more time and effort, so it is not likely that a shop will do this unless specifically asked to do so.
FWIW, this info comes from talking with my cousin who has worked at a tire shop for many years.
Also, some shops may give you a lifetime balance program. Call around, and don't make it a point to tell them you'll be offroading.