I don't think the vehicle vibrates at 2k while its sitting in neutral, but i'll reconfirm today. Is there a good tutorial for what play in the wrong directions looks like? If I get under there and try and move the driveshafts around, they don't move more than maybe 1/8 inch side to side or up and down.

 

That's the point, you wouldn't have the truck in Neutral. When you drop the driveshafts put the truck in DRIVE. This will make the front and rear outputs on the Transfercase turn, but no power will be supplied to the wheels because the driveshafts will be out.

Go on the YouTubes and search "bad ujoint". Most of the results will be examples of VERY bad ujoints. Any kind of that side to side play is bad though. And I've had a hard time diagnosing a couple minor worn ujoints with the driveshaft connected at each end. That's why I recommended dropping the driveshafts, inspecting, and running the truck in Drive to check the RPM issue.

 

That makes a ton of sense, thanks. As soon as I have a minute I'll knock that out.

 

You can also remove 1 drive shaft at a time, put CDL in lock, and drive around on just front or rear wheels, this may also isolate it.