I did a check to see if the yoke engagement was having an effect anyway. I unbolted the drive shaft from the diff and checked the play in the gearbox at various engagement depths - see this video
http://www.youtube.com/watch?v=P9w_dXOhOSs
I checked the play in 2nd gear in that video. It seems like there might be play in the gear set, and that the play was the same regardless of engagement depth. I have heard that some new tremecs may have been shipped with incorrect shimming... my research continues....