Its located inside the trans and need to open the drain plug on the transmission oil pan with a socket and drain the oil into a drain pan, then need to remove the bolts that secure the transmission oil pan to the transmission.
Now could you to remove the bolt that holds the transmission dipstick bracket
to the bell housing with a socket. You can locate the bolt from the
engine compartment. Place a scraper between the transmission pan and the bottom
of the transmission and break the seal loose so you can remove the pan.
Remove the defective solenoid by removing the bolt that
secures the solenoid to the harness. Three solenoids control the
shifting on the Jeep. All have white and yellow wiring harnesses
connected to them. The solenoid to the left (towards the Cherokee's
engine) controls shifting "overdrive." The solenoid in the middle
controls shifts between second and third gear. The third solenoid,
closest to the rear of the Cherokee, controls shifts between first and
second and between third and fourth. If you know which gear your
transmission is stuck in, replace the solenoid that corresponds with
that gear. If you are not sure, remove all three. Additionald etalis can find in this link: Jeep Secc-21S-Automatic-Transmission-42RE
Why you askw about this? Has some symthom or fault code there?