No more gears, just neutral

[06 Expie]

Help! I'm hoping this has happened to someone else and they can tell me what's wrong with my 2006 Exped.
I live in the mountains, so I was driving uphill, as usual. Grade got steeper so I accelerated to maintain speed. On doing so I heard a loud metallic clap and then I was in neutral. I have no reverse, I have no drive, I have no low gears! The engine still purrs like a kitten, but it's like I have no more transmission, at all. There was no fluid leak, no smell, no warning signs.
Husband's friend said it must be the transfer case, so we replaced it and it made no difference.

 

[whtbronco]

Well if you can still start the truck you know it's not the flexplate and we know it's not the transfer case. That pretty much leaves the transmission or torque converter and from the sound of it something broke during the downshift. A scanner that can read the transmission may give you a better idea.

Hmm, have you verified it's not the rear differential? I'm wondering if you snapped the pinion shaft in the rear diff. I did that once as a youngster. Put the transmission in neutral and see if the driveshaft will turn by hand or put it in gear and have someone look from off to the side to see if the driveshaft turns as you release the brakes a bit.