Yeppers, chances are it's simply lumps of mud throwing off the balance. I've even had that happens in cars after going through deep snow and slush. The slush and snow sticks to certain parts of the wheel and once you get up to speed sure as shootin, there's a vibration. That's when I find somewhere warm to park it and let it thaw off. After that, all is good and smooth again.

 

I took my truck for a little wheeling. Took hours to get it clean. When I took it to Ford for its checkup they said "Wow your wheel and brakes were really muddy, we cleaned that out for ya."



"Really don't know where that could have come from "

Picture is getting old but I don't know if I want to go through the mess of cleaning it again

 

And spin you could. 0* turn radius... Some say the top speed is ~30, but I can tell you as a former AAV mech, it's much closer to 60, sometimes 65. Now the AAAV's are a different story. They're supposed to keep up with the M1's.