1995 Other Oldsmobile Models Shifting into higher gears too

My Olds Achieva has started to have this problem off and on over the last month. It will be perfectly fine for a while and then start acting up. I changed the vaccuum modulator and this did not help. The oil is red and does not smell burnt, but has never been changed since I have owned the car. (40000 miles)
I seems like a manual transmission car when too high a gear is selected at a given speed. Putting the selector in 3 or 2 seems to help eliminate the shudder somewhat. Transmission shops only want to replace the whole transmission. (About $1800) I am somewhat of a mechanic and could remove the pan and change the filter and oil. I have no check engine light and the tranny connector plug looks clean.