I'm using Super S Cotton Picker Spindle Grease 00 weight from Tractor Supply to keep leakage at a minimum, and to ensure that the grease is still low enough viscosity to flow back to the bearings from the bottom of the housing after the truck has set awhile. 00 is a bit thinner than wheel bearing grease. I have no documentation that supports the idea that this is the right stuff, tho, it's just what I use.
Here's a link to the HAMB message board that addresses this question.
https://www.jalopyjournal.com/forum/threads/lube-for-torque-tube-drive-shaft.761071/
