I want to say it's for part of the spare tire hanger.
If I remember correctly when I took my spare tire stuff down it had a bolt down through that hole that was part of it...joe
I guess that's possible... but my spare tire hanger is only attached to the frame and is farther forward, just behind the differential. Here is some pics.. look at the last two or three Pictures that show the spare tire hanger from underneath.
pictures What year is yours?
