What is causing the vibrations in my Xterra

I drive a 2006 Nissan Xterra and for the last month, I’ve been getting severe vibrations when driving, even at fairly low speeds (35mph). I have new tires and when they were installed, the wheels were balanced and aligned. I even took the truck back in to make sure everything was done correctly. The mechanics suggested it could be that my drive shaft was loose. The put it up on a lift, and there was some “play” to it, so I had them fix it. Still the vibrations continue. I’m not much of a mechanic myself, and these pros can’t seem to isolate the problem. Any ideas on what else could be causing the problem?

Oh, to be more clear, it’s the entire truck that vibrates. Not just the steering wheel. Thanks.

It’s not clear to me if the symtpom was there before the tires were replaced, or only showed up afterwards. If the latter, I’d check for an out-of-round rear tire.