2015 SR5, 4x4, transmission issue? Advice appreciated.
 

I purchased this new in 2015 & it's not really been off road. It has 87k miles now. For the last 2 months I have noticed a shudder/shaking. This only occurs when I am driving along at low RPMs, usually when the transmission shifts to the next gear. It will feel like going over a gravel road/small bumps. If I floor it or give it more gas everything is fine. Plan to change the transmission fluid & see if that helps but wanted to see if anyone knows what this is likely to be?

Update 1
When I drive in S4 it does not happen anymore. Issue is most likely torque converter shudder. Plan to have dealership replace all trans fluid for $299. Will have them add lube guard & shudder fix at that time, hopefully this fixes it. I modded the 4runner because I planned on getting into offroading but plans changed & never worked out. Surprised at 87k I have this issue, never towed anything & all driving has been mainly highway. Will update after trans fluid is changed.

Update #2
Got the transmission fluid changed today, 4/10/23. Drove it to work & home, no longer notice the torque converter shudder. Dealership put the additives I provided in during fluid change. While it was getting serviced they called, got me for the battery & rear brakes, I knew I would need these done fairly soon so just let them do it . dealership also wanted to replace fluids in front/rear differential & transfer case for $500 but denied that one. Said it was due/missed at my 75k service. Owners manual never says to replace this unless "special operating conditions", says to check it fairly often though. Will check other places prices for this.

I am just bored sitting up front of the computer screen, so, let me ask you, since your information limited:
you made bunch of modifications, but have not been much off-road. OK. Many people go off-road with all that mods, and every day driving in city, but continue to keep it in transmission in "D". That "D" is tricky: every time you remove your foot from gas pedal and vehicle speed above 30MPH, tranny shifts into 5th gear (look at RPM). That apply strain to tranny clutch plates, effectively wears them prematurely. That shudder is premature wear of plates. Since you were busy with all mods, I sarcastically assume that transmission fluid weren't changed not at 30K, not at 60K. I think "D" is for highway only, in city I drive in 3rd gear, try driving in different gears on various speeds, observe and think.

Drive line shudder
 

I did a lot of research on this topic. See my older posts. This issue well documented and goes back decades across many Toyota models.
Quick reply: a faulty tranny valve solenoid is causing a Flex Lock torque converter malfunction.
Rebuilding the tranny valve body(inside transmission) is the cure. Under warranty my local dealer first replaced torque converter then a few weeks later replaced the transmission.
Also affected pick up trucks and cars.
Hope you can get it done properly.
Hungryhawk