1994 Plymouth Voyager

I am apologizing for the long question, but I wanted to make sure that I didn't leave any information out. I was driving home from work and I got stuck behind an 18 wheel truck while going up a hill. While I was behind the truck I was traveling about 35 to 37 miles per hour. As I was driving the car felt like it was shifting between gears as I was speeding up and slowing down, but at a gradual pace. When I approached the stop light at the top of the hill everything seemed fine, but when I put my foot on the gas the car got up to speed at an unusually slow rate. I know that there is something wrong with the car, but I don't know what. Prior to the incident, about 3 months earlier, I got a regular routine tune-up for the car that the manuel stated that it should be done at that mileage. However, what I know is that the car shifts between park, neutral, reverse and drive like it normally should, but the strange thing is that it doesn't seem ti shift into second or first gear as if they don't exist. The car starts fine and functions normally in reverse and as I stated before it takes a while for the car to gain speed. The car does shift while in drive and once it gets up to speed it functions normally. The only time it doesn't function normally is at low speeds, some where between first and third. I checked the transmission fluid which is about half an inche above that hot mark when the car has only been on for the recommended 60 seconds. I have called transmission specialists and they seem to think that it is the transmission, but even they are not sure because the car still moves. I want to get another opinion before I bring the car in for inspection.