I have 1996 Town & Coutry, 3.8L and it does not seem to select 4th gear (RPM seems to high at cruising speeds - I think it stays in 3rd gear). While cruising at highway speed, when I take my foot of the gas pedal, th RPM drops to where would expect it to be in 4th gear, but revs back up ans stays there when I step on the gas pedal.

I checked the transmission fluid - the level is OK, color and smell are OK, too. No codes detected by code scanner. While cruising at 65MPH fuel consuption is about 19 MPG (I used to see aout 22 MPG). By the way, what RPM should I see at 65 MPH

Any ideas what could be wrong?
Thanks.