1989 With Automatic will not downshift!

I drive a 1989 Dakota V6,2wd drive with a Automatic Trannie. I dropped the Transmission pan, installed a new filter and topped it off with Dextron II. That was at 127000 miles. Now at 137000 it will not downshift to low gear when coming to a stop. It will almost kill the motor if I don't shift into netural. I can start off in Low2 and drive around town in stop and go traffic and it will drive OK. Fluid level and color appear to be OK. Any thoughts on this? Should I have used ATF=4 Fluid? Truck runs great other than this! Thanks for any help.