Hey Guys, we have a 98 EB 4x4 with an RCD 5.5" suspension lift, 315/70r BFG's and 4:56 gears with an annoying vibration coming from the front driveshaft area when in 2wd. Switching to 4wd makes the vibration stop, as does removing the front driveshaft. I had the shaft checked at a driveline shop and all is well with the shaft itself. I do not believe it is a driveline angle problem since the shaft is approx. the same angle as stock, besides, the problem clears when in 4wd. However, it did present itself after install of the lift. We have checked all wiring/vaccum connections and the front wheels disconnect and free-wheel in 2wd. Through some testing, I have determined that the front driveshaft is turning (and with considerable force, not just spinning slightly)
when in 2wd. Seems to me that if the shaft is being turned, with no resistance at the differential end, a vibration could result. Then in 4wd, with resistance from the dif, everything tightens and the vibration stops. Why is the driveshaft turning in 2wd? Is this normal? I can understand a little movement, but this seems excessive. Anyone have info an the mechanics/operation/theory if the x-fer case? Thanks

 

As for the vibration I can't say............. The reason the front shaft turns in 2WD is to allow the shift on the fly at any speed possible. The old shift on the fly systems used auto locking hubs and stuff to allow the front axles and/or diffr. to be engaged only on demand. The problem with that setup was when the "yuppie 4 wheeler" would hit the little button on the dash to engage 4WD at (50 mph) he would hear this grinding noise and run back to the dealer. He/she did not understand the total concept of shift on the fly. The original concept was that if you had used 4WD moments ago and you had not backed the vehicle up while in 2WD the front end would stay engaged, for the next mud hole or hill. The free wheeling axle and hub concept was intended to be a gas saving 4WD solution to manual hubs. Later research and more available gas from Sadaam and the crew made the crew made the 2-3 mpg mileage obsolete. So now basically all of your 1/2 ton 4WD vehicles keep all axles engaged at all times so every thing is turning at the same speed making shift on the fly, a smooth experience.