It very well may be the ternsmission itself, but before I pulled it, I would check the lever and linkage. With that much body lift, it's not unusual for the shift lever to be slightly hitting the edge of the hole in the floorboard, at least when the vehicle shakes a little, causing it to pop out of gear.

I would take off the rubber boot and such, so that you can see the clearance between the shift lever and the edge of the hole. Then drive it like that for a little while, and see if the lever is coming close to the edge of the hole.

BTW, why do you have 11 inches of total lift just to run 36" tires? I have a total of 6-1/2" of lift for 35" tires, and I'd probably only need another 1/2" to be able to run 36" tires.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.