I have noticed a weird "thing" on my explorer ('98 4WD 4.0L SOHC - 45K). The truck seems to be leaning a bit to the left. If I measure the space between the rear right wheel to the body (from the center to the body - the part above the wheel) and compare it to the left I find that there is a difference of almost one inch. It seems like the truck is "leaning" to the left. It is less noticeable on the front but there is a difference there as well.
The truck doesn't seem to have had any accident (I bought it second hand from the original owner). I looked under the back of it and there isn't any damage visible. All looks in place. The only thing I might have noticed is that the rubber parts that connect the stabilizer arm to the stabilizer bar on the right side seem to be a bit off. But not to a level that will cause such a difference.
I will grab my camera tomorrow and add some pictures of the whole situation.
Could your rear shocks possibly be gas charged with one of them giving up the ghost? Try disconnecting both lower shock mounts and re-measure the height. If both side measurements are close, then you definately have a weak shock absorber on one side, if not then a weak spring.
Gas charged shocks will fully extend on their own if unrestrained. Most shocks nowdays are gas charged to some extent.
I have seen many vehicles with a weak coil spring or two. Only good solution is to replace them (replace them in pairs for a good match). Leaf springs generally not as susceptable to sagging unless constantly overloaded and stressed. If your is a leaf assembly, you may be able to install a spacer between spring and axle tube to raise the low side or drop the high side depending if you have spring over axle or spring under axle, but would be better to get a replacement pair.