Can I use this creeper on a sloped garage floor or driveway?

The swivel casters will roll on sloped concrete, but you'll need to chock the wheels or work on level ground to keep it from rolling away while you're underneath. Flat surface gives you the safest working position.

What's the weight limit on this creeper?

Weight capacity is not specified in the product data — reach out to support with your weight and they can confirm whether this unit will handle your load safely.

Do I need to assemble this, or does it ship ready to roll?

Ships as a complete assembly ready to use — no build required. Just unbox and start rolling.