First of all, thanks to all who gave a window into this unreasonably mysterious repair.
I'd like to offer a few more photos for those of you that have the smaller of the pantry slides.
First I removed the "floor of the bottom shelf. It was stapled. I used a crochet needle next to the 2 staples on the side I could see (the back). I jammed the crochet needle in the gap between the bottom panel and the wall, rotated, then pulled. With the 2 side staples removed I rotated the floor panel up. (Photo 1)
With it out of the way, you can see the 4 screws bolting the pantry to the floor mounted slide box.
Next I lifted the pantry off the slide box. Photo 2, 3. Notice the plastic latches bolted to the back of the pantry.
Looking inside the hole that the pantry came from (photo 4), you can see the slide rails that the box runs on, and the plastic latches that the pantry is intended to latch into. One of my latches was closed (preventing the pantry from closing.) I opened it.
I reassembled the pantry to the slide box. It again would latch!!!
As noted in order to latch it is a close fit (IE shove the bottom of the pantry to close). You can see the black drain pipe that was mentioned. If this did not latch, I was going to put a spacer behind the pantry's latches to make them extend a bit more. But this was adequate.
I felt no need to restaple the floor of the bottom shelf. Gravity is adequate.
Also I am glad I didn't try to lift the pantry off the slide box (when I didn't know how it was all attached.)