If you take off your tunnel cover you'll see this if you manage to get all the bolts off of the cover, the one in the back is a big challenge , But I got it with a universal joint , and socket and ratchet. then you'll see that part with the holes and grooves for the tab and spring and detent ball.
maybe it's something with the shifter or linkages and I also saw a post , where someone said on the little plastic tabs that goes on the shift forks fell off and he was having a problem like yours
1984 jeep cj7, 258 I6, rebuilt T176,rebuilt Dana 300, amc 20 , dana 30, 4.10's Trac-lok in rear , 4.10's detroit soft locker in dana 30, 2.5 inch bds suspension lift, 1 inch polyurethan body mount lift. Line ex. weber carburetor. factory tach. factory clock ,oem replacement speedomter cluster, and oil pressure gauge, factory volt gauge.