My truck started shaking while driving. Any advice?

What is your question?

I was driving home today I was shifting to 5th gear and my truck started to shake! I've been feeling a little vibration under my drivers seat and went underneath to see what was wrong and I noticed that there is a drive line or shaft (whatever it's called) going to the front axel and it is loose. But all the bolts were tight.. I don't know what's wrong! Does anyone have any advice?