2015 Tundra 2WD Driveshaft Yoke Play at Transmission (vibration)

Searched around but other vibrations related to drive shaft seem to be at lower speeds.
OK, here's the situation:
There's a significant vibration / shudder that's most noticeable at 40mph, though once you know it's there I can feel / hear it from 30-60ish. At first it sounded like the trans was downshifting and the engine was revving a bit high but I've verified it's not tied into the engine RPMs and still happens when the trans is in neutral. It's definitely only tied into vehicle speed.
I got the truck up on stilts and checked for bearing play on all 4 wheels and everything felt good. I wiggled the U joints and they felt solid. There's a little bit of torsional play at the diff but no more than I've felt on my other trucks.
I got to the front of the drive shaft and it wiggles a bit side to side. I started it up and engaged LSD to get the rear wheels spinning to 30-40 mph but there's so much other noise I can't tell for sure where the vibration noise is coming from, and I hesitate to crawl under a vehicle with so many things spinning fast if there's a better option...

So, questions:
1) How much play at the front of the driveshaft where the yoke goes into the tail housing is "normal?" Not rotational or slip-yoke-wise, but lateral/vertical wiggle.
2) Is there a good way to be sure it's the drive shaft that's the vibration / noise culprit? (better than crawling under the truck while on stilts and the wife tries to hold it at 40mph)
3) Assuming it's the drive shaft, and it's vibrating at the tail housing, is there a rebuild kit available (gasket, bearing/bushing, seal) for the tail housing? Assuming it's the tail housing's fault and not something further up the output shaft...

Thanks in advance!