I have a similar vibration at 65-70 as well. I too have the bushing installed and have a 3" ToyTec lift. Tried everything I could think of to eliminate the vibe, no luck. It is certainly worse when the weather is cold, sub-zero temperatures. When the weather is warm, I don't notice it at all. That in itself makes me think that it is a lubrication issue. At cold temperatures the gear oil is thicker and takes longer to start doing it's job. When the diff warms up, it gets better.
Things I have tried:
1. Tire balance - I had the problem with the factory wheels and winter tires. When I put my new Method wheels and new tires on, still had the vibe. Tried rotating front to back and back to front with no change.
2. Removed rear drive shaft, took truck for a drive on the highway. Still had the vibe at 65-70, so not a rear drive shaft issue.
3. Took out the front diff drop. No change, so put it back in.
4. Wheel alignment - Still had the vibe. Had another wheel alignment done more recently. I have Light Racing UCA's, so had the wheel moved as far forward as possible to allow more clearance at back of front wheel well. Thought this may have affected the vibe as it realigned the front CV axle a little, for better or worse I can't say. Either way, it didn't affect the vibe at all. Current wheel alignment specs are Caster Left 2.7, Caster Right 2.9, Camber Left 0.1, Camber Right 0.0, Toe Left 0.02 & Toe Right -0.01. Specs seem excellent from what I have read, so I have ruled out wheel alignment.
5. Flushed out front diff and filled with Red Line 75W85 Synthetic fluid. No change, still the same vibe.
With the fact that it is much worse at cold temperatures and only at highway speeds, I have begun to wonder if the bushing has cured the low speed vibe, but introduced a slight vibe at highway speeds when very cold. I must say, the bushing really made a difference for the vibe at lower speeds. I hadn't driven the truck in very cold weather prior to the bushing change, so don't know if the vibe was there before or not. It would also have been nice to know if the vibe existed at stock height or not.
I'm at a loss to determine what is causing the vibe at 65-70 on the highway. Perhaps someone else may have some luck and enlighten those of us who are experiencing the same issue. :-)