Just wondering, what's your objection to welding-up the diff? A full spool from OEM costs $500 and welding-up the diff is free. If you're only doing this on one of the rear axles, say the middle, and that axle would not be driven on-road (free-wheeling), what's the problem?
When the vehicle is turning the outside wheel must roll faster (More RPM) than the inside wheel. If the differential is a spool or otherwise welded both wheels will obviously be forced to make the same RPM no matter what. When off road or on very loose road conditions, (Snow, ice, mud, sand) this is not much of a problem since the ground surface allows for one or the other or both tires to slip as needed to allow for the turn. On hard surfaces with good traction, this can and will break parts.
Also, this can cause a serious plowing or understeer effect and many racers do not run a spool or welded diff just because of this.
This is the cool thing about a Detroit locker, (This applies ONLY to the DL, the other lockers all work differently) as the outside wheel travels faster it unlocks from the diff and is free to go faster than the inside wheel which remains powered (or not powered if it is a drag axle because it is currently not being powered).
To picture it in your head, think of this: You have a normal ratchet wrench with a socket on it and your tightening a self locking nut onto a bolt, (There is "Traction between the nut/bolt - You are the vehicle engine, the ratchet handle is the driveshaft, the ratchet head of the tool (The part with the gears inside) is the differential and the nut is the inside wheel) the guy holding the head of the bolt (The outside wheel) from moving is not using a wrench, he is using an air ratchet. All of a sudden he gets bored and hits the trigger on the air tool. (This is the outside wheel in a turn) The bolt head (Outside wheel) starts turning a lot faster than the inside wheel (your manual ratchet) what happens? Your ratchet (Differential) simply unlocks and the bolt (Outside wheel) "Freewheels" until your buddy gets off the trigger, (Vehicle is going straight again) and the bolt slows down to a speed equal to or less than the speed you are turning the nut.
In other words, (Assume for a moment a 1:1 ring/pinion so that 1 RPM of driveshaft = 1 RPM of axle) with a Detroit locker both wheels will ALWAYS be going AT LEAST the driveshaft RPM. Either wheel if free to go FASTER than that if it finds the need/ability but as long as the driveshaft is turning both wheels will spin at least as fast as that.
One "Problem" with the DL is that as soon as the slower powered (Inside) wheel speed catches up to the unlocked one, it locks back. If while making the turn the slower inside wheel looses traction and begins to spin, as soon as that wheel RPM matches the RPM of the outside wheel the outside wheel locks back in and begins to pull. While still in the turn if the inside wheel again gains traction and begins to pull the vehicle it will (Because it is again under load) it will slow down just enough so that it is slower than the outside wheel which is still having to travel further than the inside wheel and the outside will unlock and allow it to roll at a higher RPM again.
This is not really a true problem but this locking/unlocking action happening during a turn can occasionally cause the sensation of less than positive control on the vehicle. The lighter the vehicle the more you will get that sensation.
For instance, a guy in a Mustang that has stripped out all the extra weight for racing will really feel it a lot where a guy in a Deuce will probably never feel it unless he is really in tune with the vehicle and knows to "Look" for it.
A quirk with Detroit lockers is that some lock and unlock harder or softer than others. (This is unique to each unit (Like people, every one is different) and does not seem to depend on if it is shimmed loose or tight, it just is what it is..) This makes some people feel that the Detroit unit is harsh and ruins the ride and others to not even notice it is there. Add the individual sensitivity level (Remember the princess and the pea) and some people will really argue the fact that it is or is not good/tolerable blah, blah, blah. I can tell you that the "Worst" one I ever drove was completely tolerable to me and was actually kind of nice because if you were really paying attention to it (And you knew and understood how it worked and what was going on inside the diff case) you could tell for sure when it locked or unlocked and you knew it was working properly. On the other hand, the owners wife hated it.
Other than drag racing (Be it on asphalt or dirt/mud) or an application where the vehicle is ONLY operated off road and never on the road a spool or welded diff is usually more problem that benefit.
Running a spool or welded diff on the street will quickly tear up parts. If the axle is getting power or not really does not matter in this respect. Powered or not, the outside wheel still has to go faster than the inside wheel, the better the traction between road and tire, the more important this is.
The cool thing about the DL over an air locker is that it is always there, always working, no air compressor to wear out, no air line to break, no air valve to clog up, no wiring or plumbing to run. It is just there and works quietly in the background all the time.
Hope this helps, sorry it got so long.