The sub frame idea seems so simple and logical to me given the requirements, but I’m almost 100% ignorant of any such extreme mod and probably shouldn’t even contribute here.
But as a logical human with a reasonable understanding of geometry and physics, all I can think of in my mind’s eye to meet the requirement given is to weld / bolt a massive hunk of steel between the existing frame (the aforementioned “ sub frame” idea) and desired location of new frame mounting point (12 inches lower than current) for both the forward and aft leaf spring “connections”, in effect keeping the axle and differential on an parallel horizontal plane but 12 inches lower than it is pre-mod. The only thought would be to make the aft change greater than the forward change in order to maintain the existing (or desired) pinion angle. Aside from that the obvious considerations of longer drive shaft and shock mounts and ladder bars exist, which Pighuntin is already aware of.
It seems like what Pighuntin was looking for is for someone to take that type of subframe idea described above (which seems like a pretty simple theoretical solution) and then expand on very very specific details such as , “I’d use a 4 inch box 3/4” wall steel stock with at least a 25,000 PSI tensile/compression strength rating and bolt it to the frame using XX” stainless xx” bolts with ..blah blah..then TIG/MIG/stick weld around the seams ... blah blah” you get the point.
Any perceived rudeness aside, perhaps the miscommunication and frustration in this thread was someone thinking he was asking one thing and others reading something different. Made more complex by heated discussions that didn’t contribute to the solution. Not uncommon when trying to communicate about such tedious details on such a small screen, or when talking to a wife.
All I want to see is the final product, a 12 inch lifted SAS with 6 dead pigs in the bed pulling a 15,000 lb trailer full of used truck parts through the NM desert with no axle wrap, call me selfish ! good luck to all and god bless.