Classic/Antique Car Repair: 1971 Trans Am, crager wheels, drive shaft

Question
I'm having significant vibration from th rear section of the car between 65-75 MPH. I currently drive with Crager wheels versus the honeycomb wheels. Initially when I changed the wheels I didn't have the vibration. Could it be the u joints? How do I inspect the u joints?

Answer
could definitely be U-joints and that can be very dangerous.

It help to elevate car off the ground, ramps are great. With the drive train in neutral and NO stress on it, vigorously move the drive shaft up and down where the     u-joints are. Test both ends.

Also, a joint that bad will have a distinctive sound when changing from drive to reverse, usually described as a "clunk"