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.