Slight shuddering at 35-45 mph

[Brian L]

71 Charger, automatic. I'm getting a cycling vibration at 35-45 mph, especially at constant speed, not accelerating, just maintaining speed. It's not a constant vibration, but it cycles (vibrates for a second then stops, vibrates again, etc). I thought it might be a u-joint, but they look good and feel tight. The front u-joint actually looks new, still clean. I've only had the car for a couple of weeks, and I don't know the history of it (came from a dealer). It also makes a squeaking sound from the transmission. Trans fluid looks good.

I thought maybe it could be the stall converter. Any way to test it to make sure before I take it to a transmission shop? Any other ideas?

[303 Mopar]

Shift into neutral, this will help determine if it is the engine or the trans.  You should also check the front tires and suspension.