Transmission 1987 Chrysler 5th Ave

My son has a 1987 Chrysler 5th Ave. The transmission (auto) does not want to shift in reverse. It does go in neutral and forward. And it drives well forward. Any ideas how to fix this?

The reverse band application strut fell out. No big deal to fix. The band is supposed to be adjusted every now and then but they never are…so the strut falls out…not much more than a trans service.