Aside from the misunderstanding about grounds......
This is a typical RFI issue. The speedometer gets its information from a mag switch built in to the transmission. This information from it is used by several different systems (speed, distance, cruise control, et. al.). GM products are not any more or less effected by RFI than any other make.
I suggest you try a mix 31 bead on the connection closest to the sensor. Your dealer can tell you where it is located, or you can buy an electrical trouble shooting manual from Helms Inc. (probably a good idea for any amateur running mobile).
If you haven't done so already, a good dose of bonding will help, especially the tail and exhaust pipe ones. If you need more information, visit my web site under Bonding.