I'd have to think about this for a second, but I don't think the EZ and other similiar types of lockers unlock when you let off the gas as long as you're going in a straight line. I think the only time they unlock is when both tires have traction and the wheel speed is different, such when turning on pavement. However there could be some extra play that could put a shock load on the drivetrain. If I recall correctly, there's something like a built in quarter turn or more play in my rear Lockrite.

To me, a spool seems like it would be more likely to break a birfield because when you're off roading and both tires have traction, but niether can differentate in a turn, something, somewhere has to give. And that something is going to one axle/birfield or the other, since that's the weak point in the front end. It may not necessarily be a shock load that would cause immediate breakage, which is why I'm guessing the birfields were repeatedly stress with the front spool which caused excess wear and cracks that finally showed themselves.