not a simple answer to this question
"Radial Clearance" is the distance that the impeller must be from the wall of the turbo housing to properly operate and create boost. As the bearings wear, the clearance will decrease, not increase. Obviously, the tubine shaft will most likely tend to wear the side opposite where the most force is applied.
As to Gimpy's problem, It is hard to determine a source of the noise. I have seen turbos let go, and they start screaming, that is bearings going to He!! before they disintegrate. Yes, a runaway is a problem when the oil that is supposed to be lubricating the turbo gets into the intake. Worse yet, as stated, are the parts that get into the engine. In my experience the exhaust side wears out first due to the extremes of heat imposed on the bearings.
I would look for minor oil leakage as an indication of the tolerances being far enough out of spec to cause the noise. What is also possible is that a hole has developed in the exhaust pipe, and it is "whistling" when maximum exhaust flow is present. A hole can be in the outlet of the turbo, or even before, The j pipes are known to rot out and I know when mine had a hole it was an entirely different exhaust noise than after I fixed the problem.
If you have a known good turbo and install it, you will know for certain if the noise is the turbo or not when you start the truck up. If the noise remains, then it is a physical issue of the exhaust system. In the better safe than sorry mode I would change the turbo if I could to make sure I am not going to grenade a good engine due to a bad turbo and what can come from it.
Sorry Gimpy that there is no clear answer. trust your instincts and do what you know is right.
RL
Getting ready to look her over now.
