If you've been out mudding, the first and least expensive thing to do is look underneath, at the inside of the rims for a clump of mud that wasn't cleaned off.
The second would be tire balancing, if no problem with the first check.
After that, you should check out the threads involving "Death Wobble." Doesn't sound like you've reached that point yet, but it won't be long if you're experiencing shaking. Those threads may help head off a more serious problem to come.