2nd Gen Non-Engine/Transmission intermittent vibration

[kaffine krazed]

Truck early 01 3500 ETH DEE. At times when going down the freeway at 70mph it will start to get a real bad vibration enough to have to slow way down to keep control. I've tried to ride it out but it just gets worse. I can't get it to happen on command even if I drive on the same road at the same speed it doesn't always happen. It doesn't happen often enough to take it to the dealer. Any ideas on what to check would be greatly appreciated.