Accident? Maybe a good idea to have the frame checked.
Agreed. There isn't much that can wear out in the rear end besides the leafs and you should be able to tell if one is sagging more then the other.
But you did say it was lowered. What kind of set up does it have? Maybe the drop was put on incorrectly.
Take a bunch of pictures of the differences and the set ups and we may be able to help you more.
Sent from my twin turbo Desire using diesel fuel.