Need TCM diagnosis for a '91 Maxima SE A/T

[layedoutb2k]

I have been searching everywhere but no luck. The closest I have found is a '94 which is the same gen but I believe I read it is a little different transmission. I have tried the diagnosis using that guide but I wouldn't get any flashing lights at the end.

The problem that I have that's causing me to need this is the car will "pop" out of gear. It doesn't matter what gear I'm in, it will always do it at exactly 35mph and if it is cruising or light to medium acceleration. It will do it whether I am in 2nd, 3rd, or O/D. The fluid has been neglected by the previous owner because I just flushed it and it was really dark. There wasn't any major contaminants in the fluid, just dark. I'm hoping it's some kind of sensor type issue and not something I need to pull the transmission out because.

Thanks