Randy's Ring and Pinion was the last ones that had any gears last I checked. I can't remember for sure, but I think 4.88's were all they had left. They were getting quite pricey too and then add install kits and labor if you can't do it yourself.
As far as economically, to get 4 wheel discs, you'd be better off doing a swap. Only one company, Ugly Truckling makes a disc conversion for the stock axles and with the expense its hardly worth it in my opinion given the limited parts avaialbility for the old 60/70 stock combination.
The Dana 60 front, and a 14 bolt GM from a van are the most common swap. You'll have to do disc brakes on the rear to get the clearance for the leaf springs, unless you move them inside the frame rails.
Were it me, I'd do the swap. Easy parts availability, newer components, and stronger axles overall.
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