Speedometer and Flashing brake light
 

Hello all, I have a 2001 3 button 4wd 2 door Chevy Blazer. After I let the blazer sit for a week or so, I started it up and started driving. The speedometer jolts back and forth, the abs light stays on, and the parking brake light flashes randomly. The vehicle also shifts weirdly, it is almost as if it doesn't know when to shift correctly. I'm wondering if this has anything to do with a bad Vehicle Speed Sensor? Thank you.