I have a problem with my 02 envoy wipers they skip and jitter on the lower speeds. Higher speeds are OK. I have new wipers on them and it doesnt matter if the wind screen in dry or wet. I have a video posted here: http://bit.ly/02EnvoyWipers (http://bit.ly/02EnvoyWipers)

Had the same issue, there was a recall for the wiper motor getting water intrusion, check witha dealer to see if yours was modified/ replaced.