Switching to locking differential cost/practical?

Being able to make 3 wheels spin instead of 2 will definitely help especially if you’re stuck because the ground is uneven.
I don’t know what is available as far as a posi type setup….actually I don’t think they make that for those
There were some Detroit lockers for some of the older Rockwells though.
In my opinion what you want is an air locker which you will have full control over. Unfortunately you will need a different diff housing because the one you currently have doesn’t have the right machining and or cast features. You will also need at minimum one half of the diff case that has the spline on the end and the associated parts. You will be better off finding a rebuildable unit or one already rebuilt. You will also need one new axle shaft that has a longer spline for the sliding half of the locker

 

The Detroit lockers for old Rockwell are astronomically expensive
I don’t know how mechanical you are…..must be somewhat if you’re looking at parts breakdowns…but you can probably find a core and rebuild yourself. If you buy wisely it can be done without much money or trouble

 

Maybe….but then you’re going to need an alignment to and despite the amount of technology they have to do that nowadays it still seems to be far from an exact science
If you were already going to swap the center section the only extra part would be the double splined axle shaft
Personally I would not swap the whole axle unless the housing has problems

 

I have only ever seen one heavy truck differential with a Detroit locker type set up. The carrier was full of clutch discs. The owner wanted lower gears for their fertilizer spreader truck. They dumped the locker for lower gearing. Kerns in Atlanta hooked me up with what was needed. That locker must have been very odd because it was sitting on a brake drum off to the side when I went back several months later.
We changed a single axle FL70? over to a locker. Had to have a long splined axle as mentioned. Also had to grind the side of the bango housing to clear the shift fork. Kerns set us up with that also.
Should be someone in your area that came help you as well. However as W923 said that may not be available for your application.