Bear with me because I don't know much about cars but I just noticed a problem in my '89 Jeep Cherokee. While accelerating up a hill you can fill the car speeding up and slowing down just barely. Overall, it still accelerates it just makes a slight rocking motion (forward and back) while it is doing it. This only happens while accelerating up a hill and only in 3rd and 4th gear. I checked my transmission fluid and realized that it was low. When I added more it did not fix the problem. Someone suggested that it could be the modulator valve but I wanted a second opinion and I also don't know how to check it. Any ideas? Please help me!!