Common problem. The shift forks are probably worn and the stick jumped out of its detent. It feels like its in neutral but the forks are actually stuck in gear. You may need to take off the trans tunnel cover , remove the cap and spring that hold the stick in place and pull the stick out. Then look down the shift tower and use a large screw driver to re-align the shift fork detents. After mine did it once I started being more careful with how hard I was shifting and it never happened again.
'84 CJ-17: 17" stretch, locked Toy axles, 5.5" lift, FI, AX15, D300 w/twin sticks, boatsides & junk.