Vibration in idle

I had been told by a mechanic in May of this year that the drive shaft was a little damaged and should be replaced. Then I went to a different mechanic in my home state and they told me it wasn't really necessary so I never did. Then in early August I was sitting at a light and I could immediately feel a vibration begin while I was still sitting there. From what, I couldn't really tell but it was a similar vibration to the one I was feeling before I had all the u-joints replaced earlier this year so my first thought was the drive shaft. As I drove off from the light I couldn't go 40mph without it shaking like crazy. Local mechanic told me once again to have the drive shaft replaced. Now for my actual problem -- I just took the drive shaft down today since I'm waiting on the new one later this week. I started up the engine to see if I was still getting that vibration while idling and sure enough I was which I feel like shouldn't be happening if the drive shaft is completely down, unless it's something other than the drive shaft. Any ideas what it could be? I'm seeing that doing a trans flush has been successful for some folks.

 

check your motor mounts. The driveshaft doesn't rotate if the truck is not moving. And unless you bent or severely dented your driveshaft, or had it come apart at 70 mph like me, you should not need to replace the hole thing. The usual process for fixing driveshaft related noises or vibrations is to replace the ujoints and re-balance the driveshaft. You should also replace the center support bearing as well.

 

This. Although I have never had to re-balance any drive shaft, ever. And I'm olderish.

The key statement is it is vibrating sitting still with the shaft in or out it isn't the drive shaft. Maybe a bad spark plug or coil. Do you have any check engine codes?