strange vibration??

Hey guys, my good old rusty but trusty Silverado ( '93 4x4 Ext. cab 350TBI 200,000miles ) has developed a vibration at highway speeds. Almost feels like out of balance tires but it's not that. The tires were new 5,000 miles ago along with wheel alignment and a new pitman and idler arm. Also, just replaced both U-joints on rear shaft about 4,000 miles ago. My wife says she heard a 'grinding' noise, as she called it, at one time the other day so I thought maybe a wheel bearing. Jacked it up, spun tires by hand, checked for play from top to bottom, no luck. Wheel bearings seem fine. I'm at a loss, not sure what else to look at. Could it possibly be the transfer case? When it's in 2 high nothing should be turning in the transfer case though. I myself have not heard this 'grinding' noise as my wife put it but I do feel the vibration and it sure feels like it's in the front end but it's hard to pinpoint. Any thoughts or suggestions, I'm just not ready to retire the old dog yet unless it is a MAJOR expense.

I'd check for worn out ball joints, control arm bushings, or sway bar connections.

OK, not sure what the vibration was but it's gone. However, I finally did hear the 'GRINDING' noise my wife has been talking about and I do believe the old Silvy is screwed. Sounds like a clutch or something in the tranny. On occasion, if you accelerate hard or quick there is a noise almost like if you try to shift a manual tranny without clutching. It's just for a split second then it's quiet. The strange thing is it doesn't seem to slip at the time of the 'grind'. Also, I have noticed a few times at highway speeds it almost feels like there is a slip or the tranny trying to downshift ( but no 'grinding' noise ) as the RPM'S will jump just a bit then back to normal. Anyway, it's due for a trans. filter change so I'll drop the pan and I'll expect to see metal shavings. If there isn't any out of the norm. I'm not sure what I'll do, just drive it until something comes apart I guess...