Thanks.
Going to drain the tranny tomorrow and see if there's any coolant in it.
I'm going to attach a pressurized line to one side of the cooler, cap the other side and see if it'll hold pressure just to make sure before replacing.

Appreciate the help guys.

 

I was also going to suggest pressurizing the cooler but I'd use extreme prudence....don't just hook up to an unregulated air line. I don't know typical line pressure of a 46RE but I'd doubt it's a lot.

Also no need to drop pan - you're making this harder on yourself than you need to. If there's water (coolant) in the tx you'll see a milkshake on the dipstick. Check it while it's running in N with parking brake set.

 

It was indeed the radiator/cooler.
Fortunately, no cross-contamination of coolant onto the transmission side.
Replaced the radiator, flushed the cooling system, all seems fine now- except I've still got a high idle after replacing the IAC which was done a few days before the radiator failed- new thread on that one.

Gotta love 20 year old vehicles lol..

 

Good for you for finding the problem and fixing! And thanks for following up! I'm no help on your IAC