last night when i was backing out of my driveway, there was 2 loud clunks from the rear when i got on the brakes. put it in drive, hit the brakes and nothing. put it back in reverse, hit the brakes and the same loud clunks came from the rear. so i took off the rear tires today and everything in the drums checked out ok. went for a drive and didnt hear the clunks. just doesnt make sense for it to all of a sudden just go away. any ideas of what it could be?

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2001 XLT 4x4 V6 4.0L auto
SAS'd D44 and 8.8. dual lockers, 38s, 12 bolt h1's, some other odds and ends