The ujoints were in need of some grease but there wasn't any noticeable play in them. The slip joint was pretty dry but again, no noticeable play. Can't really speak for the balancing tho.
I'm no expert, but I'm pretty sure with an out of balance drive shaft the vibration would be pretty constant, and not come/go with changing wheels.
As for the dent in the cab. I'd be willing to bet that might have happened when you caught air on the snow run (or rather when you came down after catching air). The bed/cab move quite a bit more than you'd expect.
I'm outside your window watching you read this.