i have the same problem that everybody else seems to have, when it shifts to 3rd and 4th the car rattles and shakes, i changed the gear pressure sensors but that only helped a little and i also changed the transmission fluid 1x refill what else can i do to fix it, im planning on doing another refill next time i change my oil and then one more time to make it 3x

 

There's a TSB for this, IIRC...use the search function, there's a huge thread about this.

 

MT: GM FM Synchromesh MTF
AT: 3rd & 4th Gear Pressure Switches.. 3x3 & AT Filter change.

 

Yes, I works like a charm. Make sure when you go to buy it that it says "Friction modified" on the bottle. Otherwise it isn't the right one. I got mine at a Cadillac dealer.

 

I Put in the AMSoil FM and its 100 times better than the factory stuff

 

until its fixed, i suggest shifting in sports mode. i let go of the gas a little bit or completely when i up shift, and my car doesnt shake when changing gears.