1993 Oldsmobile Cutlass Vehicle stalls when braking after h

I recently had a tune-up (I'm not sure if that matters) and now when I am decelerating from highway driving (55-60 mph), my car begins to vibrate and shake when I reduce my speed to approx. 25 mph, then stalls while still in motion. After managing to steer the car onto the shoulder, I place it in park, turn off the ignition, wait a minute or so, then turn on the ignition and everything seems to be fine (at least until the next time I have to use the highway). The hard "shimmy" or vibration I experience prior to the stall leads me to think the problem might relate to the automatic transmission (like the car is not down-shifting correctly). It is probably worth noting that I had a fairly serious transmission leak recently repaired (faulty hose)--maybe driving with insufficient transmission fluid damaged the transmission?