3rd gen 2.7L engine shake

My truck is having engine shake/vibration at low rpm's. Mainly when stopped at stop lights/stop signs. When I put it in park the shaking goes away. When I start my truck I hear a thud. But recently it all went away no vibration and thud for about 3 days. Now it's slowly coming back again. Not sure what this could be.
Any suggestions would be appreciated.

 

My 2.7 shakes like Michael J Fox himself tuned it when I'm at a light w/ foot on the brake.
As for the thud after start..is that the air injection pump, driveline slop, check the carrier or maybe rear brakes releasing?

 

The thud definitely has something to do with the motor shaking because like I said it all stopped for a few days the thud and the shaking both. The thud feels like it's coming from the motor almost like the motor moves on startup and hits something. I'm thinking a bad motor mount but I don't understand why sometimes it does it and sometimes not...

 

Have someone firmly hold the brake down while in Drive and lightly rev the engine. If you see the engine with pronounced movement that maybe a bad motor mount.

 

this a good idea, stand to the side incase it goes ZIP!. A bad motor mount will be evident with a firm load and throttle, you’ll see it move.