Transmission

You may want to drop the transmission fluid pan and see if there is a lot of debris that is in the pan and clogging up the filter. If so, you could try changing the filter, cleaning out the debris, putting a new pan gasket on and new fluid but that debris has most likely fallen off the clutch packs and it may work again for awhile or may not, it may also work a long time, depending on how much debris there is, but it is worth a shot. Other than that, you may need to have the transmission rebuilt or another installed.

It is still worth dropping the pan if anything to see how much debris has fallen down into the pan and a new filter cannot hurt. If the fluid looks brown and/or smells burnt you may want to flush all that out and add new fluid.

Other than that, retrieve any trouble codes from the PCM and TCM to see if perhaps there could be a sensor malfunctioning. Then if no codes and new fluid if needed and/or a new filter did not help, the next step would be to check the fluid pressure. Usually there is a plug on the transmission that you can remove and hookup a pressure gauge and read the pressure to see if maybe it is a pressure problem.