I had a similar problem on my 50 with one rear drum.
I tried a mallet, pry bar.
it ended up being corrosion as seen in this
pic You can see where the black paint is corroded over.
I got it to brake loose by spraying wd-40 in the pac man looking hole on the drum face, and the two round cut outs on the drum
pic 2 Your year truck may not have those holes on the drum
Good Luck
