05 xterra has a strange intermittent vibration at varing speed the feel like your going over rumble strips in the road. Only does it for a few seconds and nothing seems to effect when or how often. Newly rebuilt rear end included everything inside and didn't seem to fix it. (rear end needed rebuilt reguardless)
Any thought s out there? All shafts ujoints etc are tight and appear normal. Wondering about internal to the transfer case?

I have a similar issue between 30 and 35 mph. This was caused by my adding an AAL to my rear suspension. This changed raised the rear end about 1 1/2". It seems that the U joints on the 2nd Gen Xterra are very sensative to the angle and this change resulted in the vibration. Don't know if this is what has happened in your case or not, but Nissan carries a kit for this, though it did not completely eliminate the vibration on my X, it did help.

Check your carrier bearing (also refered to as a hanger bearing)on the drive shaft. If the rubber bushing around the bearing or bearing is bad it can cause a vibration. If it is an automatic transmission it could also be the torque converter clutch going out (chattering/slipping). I'd need more information or description to make a better suggestion.