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Most common was the M50D, but also available were the T-18 and in 1988 only the ZF could have been special ordered in the F-150. All transfer cases were the BW 13-56.
It also could have come with a BW 1345 transfer case. Wasnt the 1356 the same transfercase but with the electronic shift? I would also bet it had the M5OD transmission. But if in doubt climb under and take a look.