1997 Oldsmobile Cutlass transmission won't shift

Transmission problem
1997 Oldsmobile Cutlass 6 cyl Front Wheel Drive Automatic 190K miles

97 Cutlass Supreme auto transmission started down-shifting by itself on the hiway, and speedometer quit working at the same time. Now it drives OK if you leave the shifter in low, but of course you don't get anywhere very fast. If you put it in drive, it won't shift out of low, but sometimes it tries to shift for a few seconds and the speedometer starts to work when it tries, and it will jerk like it's shifting back and forth between speeds. Seems to slip after it tries to shift. Will shift when first started, but then gets worse after being driven about 1/2 mile. Fluid is full, and looks and smells OK. Low and reverse work OK. Wanted the shop to change the filter, but they say that won't help, so don't bother sticking money in it, just replace the transmission. Is there anything else that should be checked such as valve body, governor, electric solenoids, etc? Problem just happened suddenly while going 65 mph on highway. I want to try the simple stuff first before replacing the transmission. I still want to check the filter. And why does the speedometer quit working when it won't shift, and then start to work when it does shift? You can be driving about 25mph in low, and it shows 0 until it tries to shift, and then the needle jumps up and back down. Thank you for any advice you can give. I have been a mechanic most of my life and understand the basics of how an auto transmission works, but I don't work on them so I don't know what makes them shift. Governor, vacuum, computer?