518 Screech in neutral

AFAIK;
If your lock-up is coupled by an electrical switch, then it can only be decoupled by the same switch.
But
I do not have the hydraulic circuit map to prove it. But
It's easy enough to prove.

Just get the rear wheels up off the ground, get her up to 30/35 mph in Drive, and hit the lockup. Then let the engine return to idle, still in gear, and still in loc-up.
Obviously, still in drive and loc-up, the tires will continue to be powered up.
Next, stuff it into neutral, but leave the loc-up engaged, then see what happens.
If the loc-up decouples on it's own hydraulically, then the wheels will slow down, and you can stop them with the brakes.
But if the loc-up doesn't decouple and you try to stop the wheels with the brakes, something has to slip, and you can bet the convertor loc-up clutch is the weakest member
Listen for the screech, telling you the clutch is not happy! lol.

The wildcard here is the Forward clutch. If it is packed up it will cause trouble. Put the trans in reverse, and make sure the Loc-up is decoupled. This will be like putting it in two gears at once. it will take excessive throttle to turn the tires backwards.

Selecting reverse simultaneously activates the Hi drum and the L/R band. and everything in the trans spins easy in reverse. But if simultaneously the Forward clutch is dragging, the whole thing is likely to loc-up, depending on how bad the Forward clutch is bound up.
In any forward gear, the Forward clutch is activated, and if it holds, you might never know it is packed-up.
In neutral, all gears are uncoupled, so again, a packed-up Forward clutch is not discovered, except for the slight extra load on the crank.
A packed up forward clutch, is probably the number one cause of a burned up transmission. I am in no way suggesting that yours is packed up. At this point this information is only to be used to help diagnose the loc-up. Here's how;
With the engine running, put it in reverse and hold her there with the brakes. If the Forward clutch is packed up, then the guts of it are now locked up. Release the brakes, and if the car moves normally, then the F-clutch is off. That's a good thing.
Next, at idle, with the car still moving in reverse, engage the lock-up. If the loc-up engages then your trans is now operating like a manual trans. So if you step on the brakes, the engine will want to stall. To stop the car, something has to slip. Either the brakes will give up, or the engine will stall, or the loc-up clutch will slip. If the trans screeches, then you can know that the loc-up is complaining. release it ASAP!. lol.

I think that's how it will work, butum, I'm no auto-trans mechanic.
I can rebuild them, and I can install shift kits, and I know the A904 well enough to have reprogrammed one real nice, but that's all.