518 Screech in neutral

AJ/FormS said

The wildcard here is the Forward clutch. If it is packed up it will cause trouble.

In any forward gear, the Forward clutch is activated, and if it holds, you might never know it is packed -up.

A packed up forward clutch, is probably the number one cause of a burned up transmission.
If the Forward clutch is packed up, then the guts of it are now locked up.

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What do you mean by "packed -up"?
I've overhauled a handful of transmissions over the years and never heard of that.
I have a 46re going back together right now that ate it's forward clutch. For those not familiar the "forward" clutch isn't the most forward clutch within the trans by position. It's actually the rear clutch, by position within the trans case.
The failure on mine was a quite sudden failure. It was fine, no sign of problem, it had sat a week or so while I had the dash out for a heater core and evaporator replacement, and the first time I drove it afterwards we went to the store, then stopped for gas. Upon leaving the gas station it started shuddering upon takeoff out of the blue. If I could baby it into 3rd or 4th/OD it pulled like expected. I probably put another 50 miles on it with scanner connected over 3-4 different drives before I pulled it (I was looking at the electronics/solenoids for the problem to start with)
My original gut - guess was that the sprag in the back of the case went out. I wasn't expecting to find the forward clutch fried. I had to hammer/pry it apart from the hi/reverse, and the plastic spacer between the snap ring and the Belleville spring was a melted glob.when i got to it, the sprag did fall apart as I pulled the gear train behind the clutch packs out. It's getting a new one of those too, I didn't even try to put the original one back together. Just swept it up and tossed it in the trash. I'm wondering if something stuck in the valve body as suddenly as this thing ate itself, I haven't gotten to pull that apart yet.