bx25d noise from front u-joint?

[frogpond]

When going in reverse, or sometimes when turning sharply in forward, and especially with a load in the loader bucket, I hear an intermittent clunking noise from somewhere up front. The only strange thing I find easily is that there is a shiny bare spot that has been scraped on the yoke holding the drive shaft universal joint. It's on the part attached to the shaft, not the front axle. I can see it because the plastic sleeve over it got broken a few months ago (probably by some debris on the ground--this tractor spends a lot of time in the woods). I called my dealer's service department about the sleeve, and they said not to bother replacing it, as I would have to disconnect the shaft to do it, and it;s really not a necessary item. It's not clear to me what the u-joint yoke could be hitting, or, for that matter, if that's what is causing the sound. I keep the tractor in 4wd all of the time--lots of loader work, and steep and muddy conditions.

I have checked the oil in the front axle, and it's okay. I'm pretty religious about greasing, too. There's no debris stuck in there. I see that one of the tie rod boots has recently ripped, but I haven't yet had time to replace it. I wonder if tire pressure could be affecting things.
The tractor has about 620 hrs. on it. I had the 400 hr. service done at the dealer.
I would appreciate any thoughts that might shed light on the matter.