Watched a quick tear down video. The external arm lever that the shift cable attaches to on the trans, is part of a rod that goes a couple inches into the trans, ABOVE the valvebody. At the end of the rod, is a metal linkage bar which is part of the gear selection. Guessing the bar broke, came unhooked, or whatever the bar attaches to failed.
If you run into this on your trans and you are positive the cable is OK, you'll need to pull the trans pan, and look between the valvebody edge and the trans case (think there's enough room) and see if the bar is flopping around at the end of the shifter arm rod. Will need to drop the valvebody to fully inspect and then go from there.
Ah well. Hate to junk the car but it's rust-rot is something else and has been on borrowed time as it is. Got 2 extra winters out of it by using a come-a-long to get the busted rear strut tower down and welded back (tires still wore evenly after this ), has about 5 cans of expansion foam filling the gap of where the inner rear fender once was (stuff works wonders and even retains water-tight sealing; the gaps it filled were inches wide w/o structure and every inch of the rear stayed dried even when going through heavy rain. Prior, water was spraying in and hitting the front windshield despite having a rubber mat over the wheel well). Still ran like a 50k mile engine despite being 195k.