Speedometer sticks on 50 mph.

I would have a diagnostic tool check to see what vehicle speed is being measured by the appropriate sensors. Then troubleshoot from there. Your sensor, wiring, driven gear, or even the speedometer head could be at fault. This can be figured out with the right equipment and testing.

“Will 'slowly' drop to zero when” stopped .. Indicative of stepper motor failure within the speedometer unit itself - - as stated - having qualified personnel perform scan test along with test drive should reveal exactly where the problem lies. IF the instrument cluster is proven defective - there are many repair services available that can save you some money as opposed to purchasing another assembly..