97 Chevy Tahoe overdrive problems

My 97 tahoe recently started to slip in overdrive. I say slip loosly as im not really shure if it is slipping or not. The best way I can describe it is if you ever got a cold chill run up your spine and you get those shudders. Thats what the car is doing. It doesnt do it when it is cold. Only when the car warms up, usually driven 15 to 30 min. Also the car runs fine in all the other gears. It also seems to happen when the car is going up a hill or has some kind of load on it. I checked the fluid (warm in neutral) and the fluid level is where it is supposed to be. At first I thought that my tranny was going bad but then I thought it might be something else. (Sensor or ect). It has the 350cc motor and is 4x4. Around 150000 miles. Thx rob