Heard stories of the front torsion bars on the F-150 and F-250 LD's snapping in cold weather, but nothing on the rear leafs.
Have you owned since new? Has it ever been heavily loaded?
I broke an entire right rear spring package on an 1.5(?) ton Chevy truck once. The load (1 piece @ 3200 lbs) I was carrying shifted to the right and then came a dip in the road. The right side went down with a crash.