1978 CJ5 stuck in 3rd gear

Had the same problem in my 81 CJ5, was a worn shift fork in my transmission. Finding a rebuilt trans or transfer case for a CJ5 can be pretty hard, it is not a hard repair. It can be done from the top, no need to drop the transmission, pull the plate for the shift boot and the whole shifter assembly comes off the top without disturbing the gears. If you can shift into neutral before you remove the shifter, do so otherwise you will need to shift the transmission by hand before installing the repaired shifter.