1968 Mustang 302 Engine Problem

I have 68 mustang with a 302auto tranny engine in it. I was driving along in it when all of sudden the stang started to shake violently i put into "2" it still shook violently then i put into "1" it still was the same. when i put into neutral the shaking went away..I was able to drive it home but.. i am not sure what the problem is...the car has 140,000 miles on it which is not bad...i really could use your guys help!

Check the u-joints. That happened in my Buick many years ago. Shook like a mother no matter what gear I put it in. Then check bushings and what not in front and rear end.