I had some x-fer case issues last February. Rocky at Marlin's looked it over, drained the oil, did not find any metal particles, gave me a clean bill of health when he was done. I had changed the tranny gear oil around that time, too.

I haven't had it in 4wd lately, I'll throw it in and see if there's a noticeable effect.

I have tried putting in the clutch on the freeway when I hear the noise to see if there's a difference. Nope, not really affected by that.

I was wondering if it could be u-joint noise? As I said the rear one is going to have to get changed out as the fitting won't take grease, and I can't get it off. The front one takes grease just fine on one fitting, the grease oozing out of the u-joint where expected, but the other front fitting only seems to take grease in the main channel. It will ooze back out at the fitting when the grease gun is attached. I can remove that fitting, and when I do the grease oozes out of the hole. I'm thinking the channels down into the joint itself are blocked. I read where the #1 failure for a u-joint is lack of lube, but what are the symptoms? I don't get any vibration/shake, just that 60mph+ ocassional grinding. Can that be the joint? I'm not sure exactly how they work, there's needle bearings in there, but can they do that? A rear end shop told me they have heard of noises being caused by u-joints failing.....