Driveline Grinding Noise at 60

Ok, Ive done the searches, posted a poll, but still would like a better more concrete answer to what might be the cause of my driveline grinding/vibration (I am assuming its driveline since its speed and rotational dependant, not engine, tire, or chasis/body related).
I noticed this if by coincidence after my OME912 lift. The noise is a vibration that can be felt through the floor, somewhat in the gas pedal, is audible, grinding type noise (sounds like gears noise) that comes in and out and at speeds primarily between 40 and 70. At first, it made the noise only when completely taking your foot of the gas pedal (coasting) at speeds from 70 down to about 50, getting quieter the slower you got. Since, I had my 90K service done, I did the front and rear diffs with Redline 75W90 Synthetic, did the TOD with Royal Purple ATF-DEXRONIII, did the engine oil, greased the drive shaft U-joints and slider, and had the dealer flush and refill the autotrans with the Royal Purple and rotate the tires (which are new with 4000 miles). Only mods other than the lift are bigger tires (P275/65R18 - but the problem wasnt there when I had these new, or my previously bigger Terra Grapplers - P285/60R18).
BUT, now it makes the noise at the same speeds, also a little lower around 40, best spot being around 65, and then up to 70 - 75, ACCEPT this time it DOESNT do it while off the gas, nor does it do it on the gas, but only when holding the speed just enough to be neither accelerating or decelerating (like try to hold a constant speed without hammering it). And you get this cyclic grinding on and off noise, sometimes constant if you hold the gas just right. This change again occuring after the fluid changes.
So , road with the dealer service rep, agrees there is a noise but not sure yet, I suugest gear noise and driveline related, but hes not convinced, it goes in next tuesday for head mechanic to look at.
Does anyone one have a solution or fix for this, or some other test I can do. Im not real confident in the dealer figuring it out and my warranty is about up!

Hows your CV boots after the lift? Any tears in them? I hear this is a common problem after a lift because of the angles the CV's now sit at. If you have a tear in the boots you could be getting some sand and grit inside the boot?

Just a thought as I'm not 100% sure what you've read, checked and eliminated at this point?

Is this a severe grinding or small but noticeable grind? Either way, I wouldn't drive it around too much replicating the noise as I'm sure it'll only get worse if you keep doing that.

I checked the boots, look OK, albeit at more of an angle. Thats my fear, driving it too much, and at highway speeds its hard to not have it do it since its at part throttle/crusing speed.
Im gonna recheck all the fluids tonight, and it goes in next week, so, who knows, maybe the dealer will find the problem!
Thanks for the suggestion!

Mine did the same thing. Stopped as soon as I took off the lift. When you lift it,it puts the driveshaft at a weird angle.I think this has happened to several people. Maybe most ,who have done the lift.

Did you add a bottle of LSD additive to the rear diff? that's a possibility, since it seems to have started after fluid change.
My $.o2, Ldub

Used synthetic in the rear that clearly stated not requiring the LSD additive, I did think about that.

UPDATE, dealer says its my rear U-joints, allthough I crawled under there before seeing the dealer and there is no play in them at all! I let everyone now how it turns out - bonus is its covered under warranty!

VR4-Quest, how did this ever turn out? Was the U-joint the problem?