HELP: Transmission Shifting Problem

[jack.smack]

A trip up the mountain caused my 1994 S/C AllTrac to give me transmission issues. Now, it won't even move in reverse. See below for context. Please advise!

I've had zero issues with this van for the past few years, but this weekend I loaded it down with a few hundred pounds of camping gear and drove up into the mountains (the drive uphill was pretty long). Everything went smoothly until the A/T Oil Temp light came on. Caught it immediately, pulled over, and shut the van off for 30 mins. The light was gone when I eventually continued up the hill, but shortly after, the check engine light came on (error code P0420). Parked the van at my destination at the top, and a day later when I went to leave, the van barely moved in reverse; I had to floor it to get it to back up slowly- and after that, reverse never worked again. It just revs as though it's still in park (I actually think it's not moving out of park into reverse to begin with). Going uphill, I don't get a lot of power out of 2 or L either. It's just been slipping a lot. The engine & transmission oil both sit exactly where they need to be on the dipsticks.

More context that might be helpful: This is the van's first rigorous drive since I changed the transmission fluid a few thousand miles ago, which I regret doing since the transmission now slips sometimes when accelerating from a stop or in reverse. I was only trying to do preventative maintenance by changing it, but now I wish I never had.

I've searched all over and I can't figure out what the issue is. Please advise. Thanks

Video Link Here. (https://youtube.com/shorts/xYuMJzmKhp8?feature=share)

[pdgizwiz]

I also have a ''94 Previa.
Some years back I loaned it to a friend to take his family up Mt Ranier. He returned and left it in my driveway, which slopes downward to my house. I started it up later and put it into reverse to move it, and it would barely move at all. I managed to limp it into my backyard where my shop is. I had recently bought a '91 for parts, so its transmission came in handy. The transmission in my '94 went to the scrap yard.
I concluded that one of the clutch packs had worn out. The van had around 270k on it at the time. The '95 and later supercharged Previas came with a stronger transmission. I don't think the older ones are bad, just not quite as robust.

[jack.smack]

Following up on this thread: my issue was indeed the transmission. First, the van cooler ruptured and interfed from high temperatures (due to a lack of tranny fluid, I believe– I think it was too low) and then the coolant (now mixed) in the transmission fluid stripped away all the clutch material. This explains why it was slipping constantly and reverse didn't work at all. It's a miracle more damage wasn't done; I didn't even get a strawberry milkshake. I replaced the radiator myself (for the new oil cooler) and then took it to my local AAMCO to pull the tranny. They rebuilt everything that needed to be fixed. Set me back more than I care to admit, but it had to be done.