why do my truck shakes when i am stopped at a stop light?

Best thing to do is to scan the computer system and see what is not working correctly. In this day and age, you MUST use a scanner to really tell what is going, with the rest of your inspections. It could be many things, so you need to have the vehicle Inspected and Diagnosed, rather than getting opinions. I would never give an 'opinion' on a running problem with out some scan data, period. It is unprofessional and basically guessing. here are some shops to speak to about your idling problem: http://repairpal.com/mazda-repair-in-spartanburg-south-carolina