02 Caravan Transmission problems

I have an 02 caravan with 84K miles, but have never changed the trans fluid and filter. It doesnt show any trany codes. I noticed several
months ago that it would not go into drive after putting in gear. I put it back into park and tried it again and it went into gear and haven had a problem until about 1 month ago it started down shifting hard from 4th to 3rd gear. I have read a couple of post and I am not sure were to start. I dont think the transmission is going out because it up shifts perfect and doesnt slip. I had it looked at (not by a transmission place) and the mechanic didnt find any codes, but put an additive in because he thought it may have a sticking valve. I have diven it for 2 weeks with the additive and still no change.
Has anyone had this problem and been able to get a good fix.
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