To access it, you'd need to remove the transmission from the vehicle. This is a major job requiring significant mechanical knowledge and specialized tools. It's not something a beginner mechanic should attempt.
If you're having transmission problems, it's strongly recommended to take your Astro to a qualified transmission specialist or mechanic. They have the expertise and tools to diagnose the issue accurately and perform the necessary repairs, which may or may not involve replacing a shift solenoid within the valve body. Simply knowing the location isn't enough to repair it.
