We have a 95 LWB Classic. The air suspension is being worked on so it is being raised manually. At about 30 or so mph we get a vibration at times, increasing the throttle usually makes it go away. The higher the rig is the better chance we seem to have to get the vibration. Another issue is that at times when you make a slow tight turn you get some popping, jerking and a bit of wheel slip, not sure if that is new or always done that, reminds me of driving a 2 1/2 ton military truck and hearing the tandem axles kind of pop in a turn.

 

Just a semi-educated guess, but I suppose I'd start by checking out the front CV joints and maybe even the transfer case.

 

Have you checked the amount of fluid in each of the front CV ball/swivels? I would guess one is dry and it is wiping out a CV.... adding mobil1 140W might quiet it down some. I would put a few squirts of mobil1 synthetic grease in there before topping it off with the 140W gear oil. Rover has "permanent" grease packs they sell for each side now instead of running the 90W, however, I don't know if they are synthetic. For $10 each side they better be... you need a super-good 1/4" combination wrench to remove the fill, check, and drain plugs on the swivel.

Good luck...