does the trany selenoid control downshift on a 1988 cutlass ciera 4 cyl and can I unplug it and still drive ok

after about half an hour on the thruway coming to an exit it shudders and stalls almost like when a clutch is not depressed on a standard and I have to put it in low to move it.driving in town there is no problem.I was told a selenoid controls high gear and is probably worn out.What was it designed to do and can I unplug it to keep the car from stalling withou hurting the transmission.what controls it and what do the two wires that plug in do.Can I put a switch on one of these wires and activate it on and off to keep it engaged on the thruway and off when coming to a stop.It seems after a mile or more in town I can shift out of low back into drive and the stalling problem goes away.