2000 Dodge Neon

My car is a automatic 4 cylinder dodge neon I bought it from a dealership with no information on it. So when I am building speed nothing odd happens. When I brake or just let go of the gas from any speed above 20 mph as soon as it gets to 19 mph the car starts shaking and making a noise. I thought it was brakes but (they are bad) thats not what is causing my problem I have already replaced all the motor mounts on the car changed the tires checked the brakes and roters. A friend thought it was the axle but another friend said the car would shake all the time if it was the axle. Someone said it was the transmission when the car downshifts into second gear. So if I am driving 35 mph and I slow down at 19 mph I have been shifting it into the 2 position on the shifter. There is (P, R, N, D, 2, 1.) And no noise no vibration no nothing so would that be the transmission then or am I ruining the trans by doing that? It shifts fine going up and down other then between 19 and 14 mph. If I don't make a complete stop and I don't shift it into 2 then accellerating it shakes and makes the same noise and it has a hard time getting back up to speed. Hope all my info details help lol