I'm hunting down a driveline vibration issue. Only occurs in neutral/clutch in over 70MPH. I have a stock aluminum DS and 4.11 gears.

I can wiggle the T56 tail yoke a little bit, maybe 0.030". Is this normal?

radially or rotationally?

Radially, meaning I can wiggle it up and down a little bit.

If you do the tailhousing bushing, some can be an absolute SOB to get the bushing in without it squishing into the oil grooves.

Also, the mainshaft/6th gear fit can be a major issue with vibration.
If you find you want to rebuild based on what you find, feel free to contact me to eliminate those issues.

But, making sure the trans. mount isn't a cause (due to excess height of aftermarket units; shim the crossmember to troubleshoot) or the driveline angles aren't causing it is the primary step, then the trans.