Cab Shaking When Accelerating??

Alrighty gents, got a bit of a mystery here...

I have an 09 TRD Sport (automatic) with just over 100k on it. It's on 33's with 5100's in the front and rear and a 1.5 AAL to level it out. Overall in excellent mechanical condition. However, I have recently noticed that when accelerating to around 20mph, and the engine's RPMs are between 1k-1.5k, the truck briefly shakes as though driving over bumps. If I continue to accelerate I simply power out of it and the shaking stops. The vibrating only occurs under those parameters. I originally thought it was it was the driveshaft and my shop agreed so they replaced the entire driveline and U-joints. That didn't solve it so we thought it might be axle wrap, so I upgraded the leaf springs, yet the problem remains.

I had another shop take it in to get a second opinion and they said the truck was in perfect condition. My only thought at this point is that I'm bogging down the engine under those conditions and it's simply shaking for a moment just as a manual transmission would.

Obviously, the fear remains that it could be something to do with the transmission, so any help to get me ahead of this would be appreciated. I will do my best to keep up with questions, thank you!

 

See what happens if you shift manually.
Leave it in the next gear down.
See if it’s RPM or Speed dependent.

That might help us to know where to look.

 

It's rpm dependent, as it only happens going from 2nd to 3rd. It could just be a lack of power on my end between gears. Only happens if I lightly accelerate in that range. Here's a link to a similar thread however I have a v6.
https://www.tacomaworld.com/threads/annoying-vibration-below-1500-rpm-while-moving.10461/