My 1989 Ford Bronco II sometimes won't shift to overdrive and won't go reverse?

I was driving one morning to work and I was caught by the traffic on the freeway. When the traffic cleared out, I sped up but my gears (automatic) stayed on 3rd gear running 55mph @ 3500-4000 rpm. I can hear the engine roaring so I had to go to the slowest lane to get me to work. When I got to work and park, I put the lever to reverse yet I had to step a little harder on the gas pedal and the gear would jerk and go reverse. When I went home, overdrive and reverse were running good again. I took it to AAMCO the very next day and found nothing wrong with the . I even had it serviced just to make sure. For about another week that the truck was running good, it did occur again on the freeway and I was so sure by the time I got to my destination, the reverse gear didn't work; and it was. Some say the problem could be the sensor module or could be the vac something.... Any ideas or clues what could be causing the overdrive suddenly won't work and same time affecting the reverse gear to not work. Any help would be helpful. Thank you.