Are there rear stabilizer bars on W123's?
 

While sitting at idle in a parking lot during the last cold snap I heard what sounded like a snowball hitting car body. I evern got out to see who threw whatever it was. Turns out there wasn't a hint of anything that hit my baby. Now when I go over a rough patch of road th rear end feels jittery and spongy. Is there a stabilizer or something that could have snapped in the cold?

Stabiliser bar link (plastic) could have snapped. It could have been other suspension components of course. What car? 300TD has the interesting hydraulic system to factor in.

Yes, the odds of the bar itself snapping would be remote. There is a plastic link on each side that is quite inexpensive and quite easy to replace. They usually fail by getting loose and rattling, but it would certainly be something worth checking.

Good luck,