8 3/4 axles 8.75 left and right differences

Axles are the same length side to side so yes you can use your other axle if it's the right length (e-body length) by swapping on an adjuster plate. As for where to buy one, used is all I know. They haven't made them for yrs. I have seen a few NOS ones pop up but generally people want a fortune for them so it's cheaper to switch to green bearings. Not sure what you mean by an "adjuster with the lock plate on it"? There's just the adjuster plate and the lock.

BTW: you don't plan on trying to use that E-body rear in your Duster do you? If so their way too wide and the spring perches are farther apart.