There are two adjustments for the parking brake, at the cable and at the shoes, see attachments. Ford recommends adjusting the cable first, and then the shoes if insufficient holding power on a grade. I think this is because the parking brake shoes are self adjusting, but Ford does not say this. Just that the shoes often wear into the drum and make the drum difficult to remove unless the adjuster is backed off first.
It is best to remove the rear brake disc that includes the drum for the parking brake shoes to make sure the shoes are good and the springs are good and there is not a leak of differential oil onto the drum/shoes from a worn seal. If the drum will not pull straight off after releasing the usual seizing of drum to hub, you can access the parking brake adjuster (see the attachment on the parking brake shoe R&I) by removing a rubber plug in the backing plate for the parking brakes. It is not shown in the illustration but it is beside the adjuster at the rear of the backing plate for driver's side and front of backing plate for the passenger side. The adjuster wheel turns to adjust the shoes. I cannot remember which way to turn the adjuster wheel but you will be able to see the number of threads increase when moving the shoes closer to the drum and vice versa.
Good luck.