Unusual engine vibration after oil change

Hey guys, new to the board here, but have seen LOTS of excellent information here.

I've been doing a search on here and can't quite find a problem like I currently have. Monday afternoon I took my '06 Titan (~170k miles) in for an oil change at a local quick oil change type place. I've had them done there before, so didn't think anything of it. When I leave the place, I notice an odd vibration between 1500 to ~2100 RPM. It's not a violent vibration, but definitely noticeable enough to have me concerned. I haven't been back to the oil change place as I haven't been able to discern what exactly the problem is. It has the vibration while in park if I rev the engine through that RPM range, so I have ruled out U-Joints. I haven't had the time to do a thorough inspection, but I have checked out the fan clutch and looked at the belt, and both appear to be fine. There isn't any slop in the clutch and all blades are there and look ok. I don't see (with my naked eye anyway) anything wrong with the harmonic balancer. I figured if any of the plugs / coils were damaged / unplugged that I would have a code showing up for a misfire. I do have 4 days off so I can actually pull the metal cover from the bottom and get a better look at the balancer.

Anyone have any ideas where to start first or have an idea what could be going on? Any suggestions would be hugely appreciated!