My F-350 is without them also and it rides like a chased rabbit when its empty down a bumpy road.
So i took some pics of the bolt-on mounts that go under the U-bolt mounting plate. These are on a D70 from a '68F-350 so they should be correct for all of our trucks.
I'm not sure how hard it would be to locate these. But if some one would like to try to fab up a pair on there own. Here's what they look like with some dimensions.
If you would like any other pics of this part, i will be removing them from this diff. in the up coming week and can post more photos if anyone wants them.
Good luck with your bouncy Bumpside.



Two-bit