Last fall just before wrapping my GT into the garage for the winter, I had a flat tire. So I went to the shop so they could fix it. I was quite shocked when i discovered that the rear spring fell of when they jacked it up.
Your rear spring may have settled a fair bit shorter than original due to sagging over time or been cut shorter - even heated to shorten it.
A standard spring is only held in by the "lumps" at each end which it fits over but does not usually fall out. Though they can usually be pulled out by had with the diff fully hanging down on the shockabsorbers.
You would have to go over a pretty big bump to get the diff into full rebound on the street so the spring would have to be pretty short to fall out under those circumstances - but anything that can go wrong will! A. Murphy again.
To remove the rear springs, you just have to disconnect the shocks and the panhard rod, drop down the differential, and lift out the springs. But if the shocks you have are longer when extended than the "proper" shock, the panhard rod may still allow the diff to drop enough to remove the springs. Not a big deal until you do what is shown below, and land without benefit of any rear springs! Hard to do, unless you live in San Fransisco and are chasing Dirty Harry.
Aside from that, how does your GT ride, and does it sag on one side, or is the rear lower than the front? If it rides OK and doesn't sag, I would say that the springs are fine.