I pulled into the driveway a while back, and the trans started to slip. I can hear what would best be described as mice clawing at metal, in park, as well as in drive. It is not a loud sound, and is not even what I would call loud. Loose exhaust shielding is louder.

It will engage firmly above 1000 RPM, forward as well as reverse, and will move at 2000 RPM, and above.

I am thinking torque converter here, as it has never had any issues slipping, was always tight in OD, and the problem began rather abruptly.

Any advice?

Update: I just got it up in the air, nothin' like less than 4" ground clearance to make that fun, and the noise is indeed coming from the converter.

 

sounds like torque converter to me too. that usually happens with a shock load to the converter.

 

I say Converter as well...

 

I will also agree it's the Converter
Pull your plug or drop your tranny pan and you may find extra goodies beyond the regular pasty looking stuff stuck to the drain plug IE: chunks/shavings.
The bad news is you have to drop the tranny to fix it.
The good news is you MAY be able to get away with a tranny flush if you can find a local shop that has the machine that hooks up to your lines and uses filters. One near me uses one successfully but it's your discretion.
Otherwise rebuild it.
Good luck

 

As a side note I'm sure you're aware that if something did let go, whatever metals got into your ATF could come back to the converter, nevermind the rest of your 700, and destroy the whole thing.
I have seen guys replace parts and not clean the tranny and lose the whole thing.
I am not saying you need to rebuild it, but you could have it flushed and drop the valve body for extra cleaning.