The brake thrashing BEGINS!!! Toooo much go and not enough whoa

[special-K]

Have you pulled the rear drums yet? Not trying to sway you from discs, as I can read and can see you are staying with a plan. But, first thing that came to mind with rear brakes locking up is you probably have oil on rear shoe r shoes from leaky wheel seal. This is a very common issue with full floating rears. I change that seal each and every time a drum comes off for that reason. Those factory brakes when in proper working order won't lock up the way you describe. Guys who want to stay with rear drums on an 8-lug truck, but want even more brakes, go to the larger dually brakes, like are found on cube vans in the junk yard. There is a thread on this swap in the 4wd section for anyone interested.