if there is no hole in the backing plate for adjusting or in the wheel hub then your're SOL.
The only way to remove the hub is to pry it off usually breaking the shoe retaining hardware and other components.

Sounds like the shoes have worn into the hub grooving it out and the shoes are holding the hub on. This is a pain in the backside when this happens and there isn't any good way to remove the hub that i know of. Been down this road a couple times on these old units....
I suppose if you knew about where the adjusters were you could try cutting or drilling a hole in the backing plate so you can get an adjusting tool in there to de-adjust it. Back then we didn't think of that, we just got a couple of big long pry bars and wrenched the hub off and then ordered all the hardware we needed to get it back together correctly.

maybe someone else knows of a way but from what i remember there isn't one on these trucks.