Low idle on brake sounds rough

Truck starts up and idles great drives great only issue is that when im on the brake at a stop light the engine starts to shake doesn't day and if given a little gas the shaking starts. I plan on getting a tune up and replacing all the filters and fuel lines as well as spark plugs.
My question is if none of this works what could the problem be? A mechanic said its the torque converter or engine mount that don't make any since though. And the car passes smog fine