1998 ford Ranger xlt sport owned since new 3.0 2x4 auto trans

about a week ago, I noticed my abs light would come on , but then go off on its own, then it came on and stayed on. about the same time , I had a brake light switch problem . the brake lights would not come on.
I then took off the little cover by the brake light switch down by the brake pedal where it plugs in and pushed on the wires, left the little cover off and that problem went away.(if i put the plastic cover back on, the brake lights would stop working, so i left it off). I also noticed the radio display would flicker until the truck warmed up. now for the MAIN problem: the truck started shifting hard into second, and also down shifted hard. So I replaced the sensor on top of the rear diff, and my abs light went off, but did not help the shifting issues, in fact it has gotton worse....the shifting has become erattic, and even seems to go into neutral by itself sometimes. the speedo has stopped working, and the OD light starts to flash as soo as it shifts hard the first time. also check engine light has came on. with my little code scanner, which is limited, it shows code P0500. not sure what to do next. any ideas????? not sure why, but I think it may be a bad connection somewhere???
please help, thanks

 

UPDATE: ALL WORKING WELL AGAIN! It was the sensor replacement on top of the rear diff. it was not seated correctly. I t needs to be flush to read correctly.