You've probably got a bad stepper motor - it controls the movement of gauges. It's a huge problem that has plagued GM products and even been the target of a class action lawsuit. There are hundreds of web references to this exact problem, i.e., speedometer pegged while standing still!

See this web site:
http://forums.trailvoy.com/showthread.php?t=48886

It's got a very detailed explanation of what's requied to change the motor out.

Good Luck,
Glenn