Having transmission rumble strip type of vibration with 2006 tacoma

Hi guys this is Toyota Lexie Guy. I am new to Toyota world this is my second posting. I wonder if anybody has a fix for this problem or any suggestions? I have a2006 Toyota Tacoma with 140,000 miles on it with a 4 L engine with a A750F transmission. I seem to be having a problem with the Transmission. There’s a vibration that feels like a RUMBLE STRIP when the torque converter locks up in fourth gear. It does not seem to happen in fifth gear. I noticed that most of the time it’s at low RPM. I’d say it ranges from 1200 to 2000 RPM and between 30 and 40 mph. It doesn’t do it every time but it does it fairly frequently. When transmissions in drive and I start getting the vibration I shift to fourth gear manually ,vibration seems to stop. (I’m thinking maybe the lock up converter disengages possibly). It’s difficult to tell because the vibration normally doesn’t last very long ,but if I try to hold the throttle steady when I feel the vibration and push my left foot lightly on the brake it also seems to make the vibration stop. (Thinking again the torque converter lockup must be disengaging) i’m thinking this problem started a couple years ago (for the longest time I was thinking it was a driveline vibration so I wasn’t too concerned )but now I’m thinking maybe a lock up converter problem ,but it was not very frequent and now it’s happening quite often. The Transmission fluid I believe without checking my records was changed at 70,000 ...80,000miles and again approx. 2000 miles ago. (142000mi). I use Amsoil synthetic ATF.
Wondering if anybody else is having this problem. And also wondering how serious of a problem it is..... am I doing more damage by driving it? I see there is also transmission additives that claim to fix problems. But I’m not sure how safe they are? I don’t want to cause transmission damage either!
Anyone have any suggestions?