Can you buy just the spring that is in the bracket?

Hi Curt, thank you for reaching out! Unfortunately we have limited replacement parts for the Xtenda Legs which is limited to a few bolts and the rubber feet. If your spring is damaged, we would suggest replacing the entire unit to maintain the proper level of safety when utilizing the Xtenda-Leg Ladder Levelers.

Are these safe on grass and mud?

For soft surfaces like grass and mud, we would recommend using this instead:
Xtenda-Leg Ladder Levelers - Cleated Feet

What size drill bit should I use for adding the bolts/screws to a sectional ladder's bottom piece?

The Xtenda-Leg Ladder Rubber Feet Levelers require a 1/4" drill & (2) 7/16" wrench or socket. After you drill (4) 1/4" holes through the ladder leg, attach the leveler using the bolts provided to the outside of the ladder leg. Additional instructions of how to install are included in the box.

Can this unit's rubber feet be switched out for the cleated feet if rubber feet are impractical for a job?

No, but we have levelers with the cleated feet.