Will these mirrors stay stable at highway speeds?

Yes. Manual mirrors lock in position once you've adjusted the crank. Vibration and wind won't shift them once set. Just avoid cranking them while moving.

Do I need any additional hardware to mount these?

They bolt directly to your truck's factory mirror mounting points. Bring a socket set and a jack; the hardware you need is already on the vehicle.

Can the mirror glass be replaced separately if it cracks?

Glass replacement depends on your mirror's internal design—contact support with your mirror part number to confirm whether the glass pane can be swapped out.