Mystery Vibration probable rear

[JustJake]

I left this on this forum weeks ago and so far no one seems to have seen it or have not experienced the same problem i have. : While in cruise control @ 75 mph I feel the vibration almost like I am on a short section of bumpy road , how ever it builds until the tire light and check eng. light comes on . If i do nothing and leave it on cruise @ 75 mph it will continue to increase the sensation aprox. 1/4 mile , but then will go all smooth and the lights go out and away right after. When i get to work we plug into it with a scanner and get no codes except for (Low ABS signal) Is this familiar to anyone ? Also when it has started to happen once previously i have sped up like you would with an out of balance tire ,Exe:If going 74 mph and when the feeling of like a rear tire out of balance starts the shake/rumble and the lights go on on the dash, I gradually increase to 78-80 mph and it goes away quicker .and the lights go out too. ( ??? )
Again, Familiar or any thoughts ?

[RX400h]

If you let off the gas and put the car in neutral, does the vibration go away? If so, have the transmission checked.

I would take it to a well-recommended auto alignment shop if you have established that wheel balance weights haven't flown off a wheel. Look for residual adhesive from weights that were on a wheel, previously. 
If by chance you have a loose transmission bolt, the sooner you have it tightened, the better.