Modern automatics are electronically controlled and quite complex. It's not something you can fix yourself, and even guessing could have 20 answers.
This is a quick run-down of the various components that could be involved in your problem:
The electronic-hydraulic control system consists of various sensors and switches, a Transmission Control Module (TCM)
and the hydraulic controller including solenoid valves. The system
controls the transmission proper including shift control, lock-up
control, timing control, reverse inhibit control, engine control, line
pressure control, auto pattern select control and shift timing control.
It also controls the AWD transfer clutch. In other words, the system
detects various operating conditions from various input signals and
sends output signals to shift solenoids
1 and 2, low clutch timing solenoid, 2-4 brake timing solenoid, line
pressure, lock-up, transfer and 2-4 brake duty solenoids (a total of