1993 Jeep Wagoneer Jerking into first gear

When my engine is hot and I come to a complete stop, wehn I accelorate, it won't shift into gear, then suddenly jerk into first. Once in first its okay until I stop again. Also, at about 55 mph, the tach suddenly drops up and down rapidly then stabilzes. I just change the trans flluid and filter but its still doing the same thing. It also feels like its being held back when accelorating. Could low diff fluid be the cause?