Wtf???

I drove my grandma's '01 durango w/a 318 the other day and holy cow get ready for all this!
1st- the speedometer won't register untill 20 miles an hour.
2nd- it won't shift into 3rd unless i manually shifted it.
3rd- I stopped at O'reilly and it would start but not idle... after 5 minutes of keeping the throttle open trying to keep it from dying it all of a sudden ran fine, exept for the other stuff.
4th- just regular braking will lock up the wheels!
5th- every once in a while especially when braking the oil guage when go down to nothin and later it would go right back up! oil level is right and trany fluid is too! the only thing i noticed was about my grandma's driving habits was she uses both feet... left on the brake and right on the gas! she acts like she's driving a stick! she gives it gas then lets off the brake, then brkes while she slowly lets off the gas! I figured that probably messed up the wheel sensor or something but what about the rest of the stuff?

PCM issue? You should have had Oreilleys scan it for error codes and then looked them up to see the real issue(s).

1st- the speedometer won't register untill 20 miles an hour. REPLACE THE REAR DIFFERENTIAL SPEED SENSOR.
2nd- it won't shift into 3rd unless i manually shifted it. DRAIN TRANNY FLUID REPLACE FILTERS
3rd- I stopped at O'reilly and it would start but not idle... after 5 minutes of keeping the throttle open trying to keep it from dying it all of a sudden ran fine, exept for the other stuff.DON'T KNOW
4th- just regular braking will lock up the wheels STICKING CALIPERS/SHOES
5th- every once in a while especially when braking the oil guage when go down to nothin and later it would go right back up BAD OIL PRESSURE SENSOR

Did all this go bad at once or did it happen over time and no one did anything about it?

3rd problem could be cam sensor- just had the same type of issue. would not stay running and blew white smoke when started. replaced this and it basically fixed the problem. When hooked up for diagnostics it read as a faulty computer, expensive mistake.... new computer did not fix the problem, cam sensor did.

wheel lock up could be overfilled master cylinder which will cause the brakes to self apply and maybe lock up, could be bad pproportioning valve which provides brake balance, or a bad metering valve. I think the durango has a combination valve and that should be checked. ( a combination valve is all 3 in 1). the tranny maybe a bad output sensor which is a screw on screw out easy job.