steering vibration starts around 70 mph

Im looking for some help here. I have a '10 Tahoe 4wd with around 60k miles on it. It was fine rolling on the stock 17"s. I put a set of 20"s with Yokahama Avids on it and I had a slight intermitent vibration that started around 70. I had them rebalanced, rotated, etc with no change. The vibration gradually got worse, more often and stronger vibration. I put the stock setup back on and it was much better, almost unnoticable. Some 6-8K miles later, the vibration is getting much worse. Rebalancing or rotating has no affect and tire shop doesn't know what to do. I should also add that when it first started, it was more noticable when making small steering corrections to the right. If there was no steering input, it was fine. Now it is all the time. I checked the tie rod ends and don't feel any exsessive play. I'm a pretty good DIY mechanic but this one has got me scratching my head. Anyone have any advise?