I know this isn't the best forum for this, but there is lots of thinking outside the box on this board, so here goes.

My 2006 mega cab 4x4 auto (3500 SRW) has developed a strange vibration over the last year or so.

Basically it is a vibration that starts around 65 MPH and gets progressively worse as speed increases. Around 70 MPH it becomes a vibration the comes in 1 second "waves". It vibrates-stops-vibrates-stops at about 1 second intervals.

Truck is very low miles at 50,000 original miles. I am the original owner and this truck is almost 100% stock with 2 exceptions: 1 piece aluminum driveshaft and 285/70/17 Bridgestone tires. Never had a tuner on it at all and has never been off-road or abused in any way.

Vibration is speed related but it is worse if under power, as opposed to when coasting.

Things done to troubleshoot:

1. Changed tires. The originals were dry rotting so put on new Bridgestone Revo tires 1 size bigger than stock.
2. Changed out 2 piece rear driveshaft. When this vibration first started my first thought was u joints or center bearing-Great excuse to upgrade to a 1 piece aluminum driveshaft. Had one built by a local shop with new u joints.
3. Removed front driveshaft.
4. Rebuilt transfer case with a bearing and seal kit. Never replaced the chain because it was still in good shape. All internals looked in fine.

My next thought was maybe the engine/transfercase mounts are getting weak, but I don't want to just throw any more parts at it.

Any help here would be great.