I have a 2004 chevy trailblazer ext with 90000 miles...
The speedometer has messed up... bought a used one out of a junk yard.. put it in and it lasted a day... speed hung at 80mph.. what is causing the problem with the speedometer... how would I fix this don't want to buy another one and have it mess up also??

The actual fix that is needed is an updated speedometer drive motor (internal to the instrument panel cluster) installed and calibrated to your vehicle. Dealers in my neck of the woods are doing this daily for less than $500.00.
The parts necessary aren't sold to the public due to the laws regarding odometer tampering.
I'd be surprised if the junk yard sold you a used unit without violating at least one of those laws.