my 02 bravada runs very sluggish at low speeds but evens out at high speeds. have changed the spark plugs and the coil pack keeps coming up with a reading of 05 on computer

You have possible faulty ignition coil, or maybe spark plug problem. Which maybe causing what seems to be a misfire. Try finding someone that you know that has a code reader they can hook up and see what codes come up. Local auto part store might be able to help. Get the code or codes and then look them up online. Might turn out to be what is common with GM's is it could be you will need to replace the Cam Position Sensor. Hope that helps

Have you changed the air and fuel filter ? Also may want to add some fuel injector cleaner to gas tank to help clean out the gum and carbon build up.