Skatehard96

I need help with my 1992 Toyota 4runner. It is an automatic trams and has been having some troubles lately. It doesn't like to shift into overdrive, but it will. It gets up to 4000 rpms and stays there for a while before it will shift. All other shifts are fine it's just into overdrive. Today when I was on my way home from work it shifted to overdrive how it usually does but it kept downshifting me as if I were going up a steep hill and then shifting back up( all at a constant throttle). It did this over and over again all the home until I just turned the overdrive off. My o/d light flashes and the check engine light is on. I can't read any codes because the plug on the car is full of gunk. Idk what the p/0 did but it won't read any codes. At first I was thinking it was vss but now I'm not so sure. Odometer and speedometer don't work.