He did not elaborate and I didn't pursue that question, however upon inspection of the joint when I was at home under the truck I did see a small pin hole (tiny tear maybe) in the rubber boot/seal that could have very possibly been an injection site now that you mention it....

 

I went out and took another look and the u-joint itself is for the most part is clean of the grease and the "wet" area is at the end of the yoke coming out of the tranny... it has a small hole (weep hole?) and this appears to be the source of the oil/lube/fluid.

The red line points to it in the picture but doesn't show it from this angle...

I did check the seal again and it appears to have a small cut in it probably from road debris but it is minor and is not weeping or leaking in anyway and is only on one of the cap seal....

 

Not sure how long that'll last.

I'd keep an eye on them.

 

My 07 tahoe was making noise when it was in drive but not in neutral. I took it in
today and it was the u joint. Its under warranty if your under the 5yr 100000 mile
power train warranty. It was done in less than one hour. That turned out good!!!