93 jeep grand Cherokee Laredo 4x4 with 4.ol straight 6

So it runs great drove ok then started coming out of gear
on freeway or Not shift up to a higher gear... I put Lucas no
slip tranny fluid in the dip stip hole.(about 3/4 of bottle) it
got better at acting like it should not missing gears or any
thing I parked it drove fine for anout 20 mins on freeway
and then wouldn't reengage into gear. When it cooled of
from sitting there it then drove briefly before repeating itself
not driving forward. I'm good with fixing things but haven't
the slightest clue about transmission issues. Any thoughts
???? Please help oh and the check engine light not on and
the only code it gives is 12 any input would be appreciated.