This is just a guess, but I think it's part of a levelling kit. Bronco's originally had a rake to the front of about an inch and a half. To level it the smallest spring for the front you could get was 3" higher than stock (I'm guessing) which requires a small lift block in the rear. Like I say, I'm not sure but when I bought my bronco, I was told it had a 3" lift on it, and it has the same block in the rear as yours does.