Had a similar problem with my 4R70W in my 94 Cougar. I was doing 50+ mph and it suddenly downshifted to second gear - it certainly got my attention. Turns out the transmission ECU thought it had been shifted to 2 and complied
. Best bet, get it fixed quick. That downshift could have made my rear end skid out if it happened when I was taking a corner or if the road was wet. If I’d been going faster it could have blown the transmission or engine.