My 2002 chevy trailblazer won't go over 3500 rpm it idle rough when in gear with brakes applied

Fuel mix? new fuel filter? throttle body injection? gasket to it OK? (there have been issues). cheaper grade spark plugs? just listing most common causes of rough idle complaint. also try manually cleaning throttle plate and tweaking up throttle angle screw (aka minimum air rate adjustment)

The first thibg to do is have the computer scanned for codes, these codes will help in finding the problem. its sounds like the truck is misfiring, this may be due to a bad sprakplugs or bad coil in one of the cylinders, but by scanning the codes they will point you in the right direction, if you get the codes and give them to me i can help you better. hope this is helpful.