Weird vibrations in drive

Hey guys, I appreciate all the info you guys have been giving lately.

Now is just a concern. Recently I've been noticing that every so often when i'm at a complete stop and at idle the car has a really really weird and strong vibration. It is almost like something on the gear is off or something is on it. Whenever it comes around thats when the vibration happens. When i put the car in neutral it goes away, when i rev it a little in park it goes away, after moving forward a ways and stopping it goes away. But it comes every so often which is worrying me.

Another concern is i feel like after i take my foot off the gas pedal if it was at 2000 rpm and i take my foot off the pedal it'll drop to 1500 instead of maybe 800, and I can hear the kick down click when i let go of the gas pedal. The kick down has been going on for a while, just didn't think much of it. The sticky pedal as i call it was after the accident and so was the vibrations.

It's a c4 auto.

Recent changes: changed my speedometer cable, added some trans fluid.

I'm not sure if its the U-joint, the car is at a standstill.

It's possible it can be the flex plate, it needs to be replaced, its missing some teeth.

I drove the car for a bit after i got it back from the body shop and it seemed fine. It was only after I changed the speedo cable is this vibration occurring, but it maybe possible it happened before. The vibrations can be felt if you put your hand on the shifter. They're pretty distinct.

Ugh, so i took it back to the body shop that did the body repair, for a few things like shifter linkage adjustment and the fuel sender was completely off, so they'd have to fix it. They said the shifter linkage adjustment they can't do, and it looks like the transmission mount is broken would that cause this vibration?