Grinding/Growling Noise Only in Reverse 6spd Manual

My 05 Tacoma is making a low grumbling noise when I let the clutch out in reverse. It only happens in reverse and is louder when the engine is under load. I jacked up the truck and put it in reverse with the wheels off the ground but the sound went away, ie it’s only when the engine is under load. Here’s a video with sound: https://youtube.com/shorts/26wtLzERIDA?feature=share

It’s the same in 4wd and 4lo. Coasting backwards in neutral makes no sound. Any idea what’s going on? I really hope the transmission isn’t going out. I’m supposed to go on a Moab trip in 2 weeks…

Update: the transfer case moving under load and rubbing the aftermarket skid plate where it hit a rock and bent up a little. Disaster averted.