The differential terminology has really gotten confusing, so I'll define the terms the way I use them.
Open diff = spins the wheel with the least traction.
Limited Slip = applies X amount of torque to both wheels (usually 250 ft lbs, or so).
Posi -Trac
Trac-Lok
Power-Loc
etc
Locking diff = when locked, solid connection to both wheels.
ARB
E-Locker
Spool = solid connections between the axles, all the time.
And then you have things like the Detroit Locker, that is not a locker (allows one wheel to turn faster, but none to turn slower. These are normally racheting gear type diffs. The lunchbox lockers work the same way, although there are lunchbox L/S's also.
And then there are the Lockers that are manually locked (flip a switch, pull a knob, etc). They really should be refered to as "Driver controlled locking diffs", as they have been in the trucking industry for years. And at least one works as a L/S until locked.
I agree that a locked diff (driver controlled) will have the same problems as a spool when locked. The difference is that the driver chooses when to lock it.
I don't understand what the concerns over shock are? In a racheting (overrunning) type diff, the out side tire turns faster, causing the gear to move forward one tooth. Perhaps the shocj your are refering to the effect (noticable on a Detroit at least) of the gear overrunning? That is not a shock to the drive line, it's merely the spring pushing the gear back (re engaging). Or possibly the "thunk" when the gear finally rachets (caused by driveline windup being released).
I guess it all boils down to, you get what you need for the conditions you drive in. :thumbup