There isn't enough space between the 4-speed and the transfer case to put in an overdrive, even though they are a divorced setup.
You MIGHT be able to fit a Ranger overdrive (Advanced Adapters) between the motor and transmission but it would significantly shorten the divorced driveshaft unless you moved the divorced transfer case. This would cause you to modify the length of both driveshafts. You MAY be able to convert your 4 speed and transer case (probably need to swap to a later NP-205 if you have the Rockwell T-221) to a married style, this would give you room for the Ranger and wouldn't cause too much change to the driveshafts.
You could possibly put a overdrive after the transfer case, but you could only use it in 2wd, and you would have to modify the length of the rear driveshaft. I don't know if those planetary gear overdrive will mate to the output of a transfercase or not.
