Shift problems in 94 GT (HELP?)

Hi, Newbie here and new to Mustangs too, At around 40 to 55 MPH my GT acts like it going into passing gear. It doesnt do it all the time..

Im told Speed sensor, and then told its the ABS sensor that the computer pulls up. Left rear sensor.
Car runs great but dead on take off, sluggesh, Shifts good except for the kicking in and out affect some times.
Some what slow on shifting to second, Automatic..

Thanks in advance guys...