rumble problems

proud one owner 230k 1st gen. tundra owner, has been an amazing truck and a rock for me since I was a young man. I now have my first mechanical issue and am coming to this page for advise. Problem started about a month ago, as soon as I shift into fourth gear a harsh rumble kicks in but once I take the O/D button off it goes away, at first thought was maybe tires or ball joints, like a suspension issue. Then I thought maybe driveshaft issue which I don't believe it to be now either, but could possible be but doubtful because I have inspected the U joints and bearings seems to be find. Plus once I hit the O/d button and it goes away makes me suspicious think its not the drive shaft. After reading some forums and thinking could be a solenoid for torque converter ? any ideas or insight would be great thanks.

 

There were problems with the OD planetary gear set in the early trucks, possible that it's on it's way out. A visit to the shop might be in order.