Do these mount using the factory headlight brackets?

Yes. These are designed for the factory mounting points on your truck—no adapters or custom brackets needed. You'll remove the old headlights and bolt these in place using the same holes and fasteners.

Will these work if my truck is stock or already modified?

They work with both. As long as your electrical connector and mounting location are original or standard, these will fit. If your truck has heavily modified headlight wiring, test-fit and verify compatibility before finalizing installation.

Are the LED bulbs replaceable, or is the whole unit a sealed assembly?

Sealed LED assembly—the bulbs are not user-replaceable. The entire headlight unit is designed as one complete piece, so if the LEDs fail, you replace the full unit.