Rebuilding the case yourself is pretty much out of the question. There are two problems with this. First is that getting the piston back together without the special tools is virtually impossible. The other is that the case itself wears out where the seal rides. When they are rebuilt the case is sleeved.
The best answer for this system is to just maintain it with the leak. Simply keep the hydraulic fluid topped off and the system will continue to work as designed with the leak. Then all you have to do is service the case as it gets full of hydraulic fluid. I maintained several 4 matics this way for several hundred thousand miles. I liked to service the transfer case when the hydraulic fluid had dropped a half quart.
Then to stretch this time frame in the summer or good weather you can flip the service valve and the system shuts off. Or on the 92-93 models move the electric switch to the test position. The car will operate in only two wheel drive. When the weather gets bad then move the switch back. Sometimes you can get a code for the stop lamp switch, which can easily be erased by either reading and clearing codes or unplugging the control unit for about 10 seconds.
By the way I love this system for the same reasons mentioned before. If you have good snow tires on this vehicle it is virtually unstoppable.