your regular obd2 scanner wont pick up trans codes, the hi end ones will tho. best to have a trans shop scan it, some charge unless they do the repair work.
so when it shifts into o/d is it chugging with a rpm fluctuation of 2 to 500 rpm type of thing? that would be a pcs solenoid.
also if you are in over drive and tap the brakes while still giving it gas, the tach should jump up 200 to 500 rpm as well.
as far as the shiftier, it may need a new cable or your shiftier itself is taking a slow death. you can always look at where the cable snaps on to on the trans, its right below the master cylinder. make sure its on and not lose or something.
