Happy to start a thread on this if we're taking over the OPs. So, the metal locating ring in the axle housing is there to help align the axle shaft at the diff end on the axle's long side (left) when it's inserted and it should never touch that ring again after the axle is fully assembled. (Public Service Announcement: Some guys have dislodged that ring by shoving the axle in and slamming into it, not knowing it's there. It will not take a hard blow from a heavy axle, which is like a pile driver when you think about it. Be gentle with that thing). I think you knew all of that, just clarifying. In terms of accuracy, there is no need for thousands of accuracy. You could theoretically shim the axle 1/32 too low and the axle would merrily spin in the seal. All that happens is this would cause it to fail early - perhaps 15k miles later and start leaking. All I did was guess how high my axle was spinning, choose a shim combo to move the axle downward that amount, assemble the trunion bearings and check for bearing tightness, binding, slack. No axle shaft in place.
To your correct point, the spindle and more precisely, the splined axle drive plate on the tip of the spindle determines whether the axle shaft rides high or low in the seal. Since the spindle is bolted to the birfield housing, lowering the housing is synonymous with lowering the spindle and thus lowering the spinning axle shaft. That's it - no rocket science needed. Years and miles later, when I pulled the seals, the lack of a discernible wear pattern on the seal lips (there are little chevron oil directors on the lip that wear off readily) and zero leakage indicated I'd essentially nailed it. Don't be impressed - it was not only easy to guess, it was also hard to screw up. If you had your birf apart in front of you right now you'd go "Oh, yeah, that Idaho guy's not that smart - piece of cake, I see how that works".
Toyota's intention with the 80 was to use modern machining to eliminate the variables on the birfield design that required previous generations to use shims, and the SST to accomplish it. They got close enough they were satisfied the 80s were coming off the line consistently running true enough for the seals to live for their interval (can't recall - 60k miles?). I just took that a step further and made mine better aligned in the center of the seals.
And of course the presence of shims on some (not all) 80s indicates there was in fact a tool at the factory for them to note the variations and adjust for it with shims. The FSM is designed for the vehicle after it's out of the production environment (factory), so naturally it would be silent on adjustments here. But real life happens, such as people bending their axle housings slightly with all the heavy wheeling, larger tires, and regears this amazing chassis is sought out for. And so today it might be nice if someone who's done that were to know they might be able to adjust for a slightly tweaked axle housing. Make sense? (PS - this begs the question "How are shops telling people their axle housing is slightly bent if there is no known tool to check where the axle shaft is running in the housing - are they just guessing and taking our money for their parts cannon?)
A bit of 80 lore for ya....
The one mystery I can't account for here is what happens with the swivel bearing preload? Someone's going to ask, and I just can't recall if there were paired shims (under the bearing itself to preserve the "stack height", etc) or what I did. I know I assembled it with and without my changes and was satisfied nothing got screwed up, and of course 160,000 miles with a lot of heavy towing, etc and all is well indicates success. Next time I repack I'll post up about it.
