what could be causing my 2004 nissan altima to shake when at a complete stop?

it could be a belt. maybe your timing, balance, or serpentine belt

many things. perhaps a torque converter problem or a transmission problem

you have worn out motor mounts and a perforated muffler. open the hood of the car with the engine running and have a friend rev the engine a little, if you see the engine rock or make thunking noise, you have a bad motor mount. to check the muffler, make sure the car is off and cold, get a flashlight and look at the muffler, if you see any black soot spots, the muffler is leaking, this is common, specially around the welds.

idler arm or engine mount and/or bushing.

idler arm or engine mount and/or bushing.