I have a 97 Tbird. I am having a couple of problems

1. When I hit around 65mph i start getting a bad vibration in the rear of my car. If I hit the gas it will go away. But then come back. It kinda intermittent. I know my rear pass wheel has a small ding in it. But im not sure. I was thinking it could be the universal joint on the drive shaft by the shop down the street says it would do it at more speeds then just 65. So Im lost.

2. Also I have just replaced my AC controll and I cant get it to switch from defrost to panel or floor. I replaced the original because of this reason. Im sure its some stupid vacume leak but I have no idea where to look. I have looked behind the glove compartment. Im lost someone help....

thanks