Hello
I have a 2004 nissan altima. I have delayed shifting and the engine used to shake when I was changing gears. I was told that motor mounts are weak and I bought 2 motor mounts from ebay (front and rear)
and gave that to my mechanics for replacement.
After Replacement, I feel more vibration at idle ,
even when I roll up the windows car shakes and rpm drops.
What should I do? The mechanics believes the motor mounts are creepy. I wanted to feel less vibration
and now I feel much more !!!

Not sure if you got this "problem" fixed but the motor mounts, I believe, is supposed to help tighten things up...specifically the motor itself. With that said, when I installed my RL MM, the dash had more vibration. Although it wasn't violent, it was very noticeable. I'm thinking that MMs would bring about vibration to the car as it holds the motor while things inside are rotating. but that energy would transfer to the rest of the car, which explains the vibration. So I found it rather normal, and not intolerable. Would I prefer it to be smooth? Of course. But I also didn't like how flimsy the car felt when I accelerated.

I think you should replace all 4 mounts including the transmission, unless you had earlier identified a completely defective mount. Even then it's probably best to replace all of them if there's lots of miles on the car. Also, two spark plug wires on the wrong plugs would cause vibration and lack of power. Did you have plugs changed right before this started?