The axle body itself is identical between 3600 and 3800
There maybe a minor detail where the springs sit on the axle
I don’t recall if there is a wider pad welded on the 1ton axle where the spring pack sits
the backing plates and drums are different sizes
If you need a new axle you can take the one from the 3600 but then you need to swap backing plates and all your brake parts
Or you can run the whole 3600 axle and brakes on your 1 ton
However that’s confusing to the future people who will work on your truck finding smaller brakes where they didn’t expect to
The 3/4 ton brake drums aren’t any easier to source so there is no advantage to doing that swap
The pads are available new for both 3600 and 3800 rear
-s