1993 Ford Thunderbird Drivetrain vibration 1993 Thunderbird

My thunderbird has a low pitch, loud vibration that seems to come from everywhere under the car. It used to only do it at highway speeds, but it's getting worse and now I notice it starting around 35 mph. At highway speeds it sounds like you are going over a metal grate bridge. It's strong enough to make my rear view mirror dance and steering wheel vibrate. Taking my foot off the gas makes it slightly louder, stepping on the gas makes it slightly quieter. Taking it out of overdrive and into 3rd makes no change. Putting it in neutral and coasting makes no change. Turning/cornering makes no change. Braking makes no change. My tires were recently balanced and show no unusual wear. There is no play in my ujoints. The transmission shifts fine through all gears and does not seem to affect the vibration. It had an alignment recently, and everything in the front end is tight. I'm thinking something in the differential? Maybe rear wheel bearings?