If you are talking about the plastic in front of the rear tire (floorboard), a hair dryer will work. it will also work on the rear mud flap on very back.
I had this problem on my rear mud flap after I removed my lift. instead of heating it, I just trimed off about 2" of it.
I had this problem on my rear mud flap after I removed my lift. instead of heating it, I just trimed off about 2" of it.