My 2000 automatic XE King Cab always shifted a little hard from 1st to
2nd. I replaced the filter & fluid twice, but that never helped the problem.
Recently, after downloading PDF copies of the factory repair manuals, I checked and reset the timing according to factory procedures.
The timing was retarded 4 degrees.
After resetting the timing, it seems to shift a little smoother. I don't know the correlation of the engine timing and shifting harshness, but the throttle position sensor can be the cause of hard shifting, but I never had a check engine light on.
Hopefully, it won't go back to shifting like it did.