I say change the fluid... if at the very least to drain out any metal shavings and to see whats inside. Drop the pan of course. I think its too late to use a thinner fluid if its internally out of spec. I cant see the gear oil, silicone, and razor blade costing more then $20 and an hour of your time. I say drain the transfer case too.

If it makes no noise stopped clutch engaged in N, it's not the input shaft. It may be the mainshaft bearings, tcase or outputshaft stuff. I'd suspect the tcase gears or chain.

I'm going to check the transfer case oil level as well, and document all the noises in all the gears, incl 4lo and hi. I'll get back to you.

Pulled the truck out of the garage and did some testing. I put the transfer case in neutral so that I could run the transmission into the gears without any road noise. What I thought was only happening in gears 1-3 is actually happening in all gears. Its a somewhat big rattling noise that happens any time the clutch is engaged, and in any gear. When I disengage the clutch, the noise goes away. I put some pressure on the clutch pedal to get some pressure onto the throwout bearing, but that didn't make a difference.

I think that all this points to the input bearing, which it's my understanding can be replaced without tearing the whole transmission apart, is that correct?

To change the input shaft bearing the wrong way... aka without taking the tranny apart you still need a special puller made by Miller Specialties. Costs over $200. Your not always guaranteed to get it out.

The ghetto way involves two screw drivers and a hammer. Again, doesn't always work. I wouldnt play this game unless you have a used spare or have the funds and are prepared to buy a Kormex. Otherwise once you half way it no tranny shop will take it on.

Your playing thr same game I played a few months ago. You got high miles, not the original owner, its a weak tranny to start, and on. Drop the tranny, verify for sure if its the bearing. If it is, just get a replacement.

In the time I spent researching and what not I could have done some man with van gigs and made the money for the tranny. In the end I had to buy it one way or the other.

Also forgot. Just because you address the bearing doesn't mean it fixed your problem for the long term. The bearing plate gets beaten out and so does a lot of other stuff. Things are no longer in tolerance and flex/bang on each other. Just a matter of time before it blows another bearing. Mine was bad enough that it broke a synchro ring and I found it in the pan.

Go to starquest so website and hit up Proffesor Quest's tranny section. Tells you how to check your tolerances and assess what your dealing with.

Untill you drop the pan your only guessing.