xterra vibration/shutter during shift between 45-50mph

Hi all,
I have just began noticing a brief vibration / shutter just during the shift at around 45-50 mph. I have a 2001 nissan xterra 4x4 with 90,000miles. The other gears shift without any problems. 4x4 works fine. could this be a transmission problem? I have even had someone tell me that it could even be related to my rear shocks. Please help.

Thanks.

Automatic transmission I presume? I'm thinking perhaps a problem with the lockup clutch for the torque converter, that's the speed that it tends to go in and out of lockup.