99 Nissan Altima GXE transmission maybe?

My 99 altima gxe has reached its 55,000 mile point I recently changed the transmission fluid and cleaned my fuel injectors. However after I start my car the rpms start off at 2,000 rpm's instead of 900. Then when I attempt to put my car into reverse or drive it begins to shake and the rpms are at 500 or less and as the car shakes the rpms are jumping all over the place below 1,000 and then most of the time the car will shut off very rarely it will recover and stay steady at 900 and theni can actually drive it I believe this could be the transmission. Also wheni switch to reverse or drive the car seems like it has to fight hard to switch over to reverse or drive. My car has the stockengine its a 4 cylinder engine and its automatic.