5th to 4th downshift bucking/stumbling shift uphill 35-40mph?

After spending several days scouring old threads for info I finally decided to sign up and ask a specific question myself in hopes someone else finds this thread and finds a resource.

Got a new-to-me 07 DCLB with 185k, meticulously well-kept, oil changed every 3-4k miles. Unfortunately status of ATF changes is unknown. I could only assume it's factory oil.

High-level--I realize this is likely a transmission/torque converter issue but maybe, hopefully, I am wrong or there's a fix apart from a replacement torque converter or tranny rebuild.


Ironically I can't reproduce the issue when the tranny is cold in any RPM range or hill. It's only after everything has warmed up can I start somewhat predictably reproducing the rough bucking/jerky shift from 5th into 4th when feathering the accelerator starting up a grade around 35-45mph. I notice a similar shudder around 65 downshifting at highway speeds but nowhere near as violent feeling.

1.) Obviously no dipstick so a bit unrealistic to check ATF fluid unless I am planning to drop the pan. With this many miles consensus seems to be leave it alone.

2.) Tried the sticky thread about the "torque converter shudder" tapping the brake but it's not so much a sustained vibration as a really "kicking" type lurch shift into 4th when gently accelerating up a hill or even slight grade. Seems like a different issue.

3.) No issues shifting in/through any other gears

4.) Problem goes away when going over to the "4" from Drive.

Any ideas? Tempted to try GearCruncher's LubeGard but again with this many miles I'm really hesitant to start swapping tranny fluid and adding stuff. I got this taco because my Subie blew a timing belt last month and was forced to buy while I rebuild that engine. Last thing I need is to blow up the transmission on this.