My 88 300SE vibration at 40 MPH

[carnut]

I have a shudder, vibration in the car at approx 2000 RPM which matches 40 MPH. The vibration is very slight in the floor of the car. If I turn a hard left turn leaving a stop sign, the vibration quits until the car is parked and starts up again as I get to 40 MPH. Some days its more pronounced than others? I'm thinking rear CV axle but which one? Turning left loads the right axle but slightly stretches the left. It's as if the joint is in a new position after the turn. If the boots are intact how do you confirm a binding CV joint.