"94" Labaron shifting to OD at low speed

I have a 1994 La Baron with the V 6 engine and OD trans.

The car accelerates fine but goes into OD at about 30 to 35 MPH. When I
attempt to accelerate at that speed the engine / trans vibrates until it
down shifts which takes a few seconds.

I tried driving in "3" but it seems to stay in what seems to me to be 2nd
gear.

Is the trans controlled by a computer or is this a linkage adjustment
problem?

BTW the car has 55,000 on it.

Thanks
Frank