I just purchased a new 2005 Xterra Off Road. I love the vehicle but the stock shifter has a "rubbery" feeling. Is anybody familar with a bushing upgrade for this transmission?

I know what you mean. I'd be interested in this also.

Another question: Do the synchros in this thing ever loosen up? It feels like I almost have to rev-match on every upshift and downshift to get it to shift smoothly.

Ya I have the same problem with the 6-speed in my 06. Not much you can do about it except cut the stick off 1-3". I haven't done it as yet but it's comming. I cut the stick off of my 02 and it cured the "rubbery" issue. I ripped the stock knob off of mine the day after I bought it and installed one of my Momo knobs. That helped a bit but the stick must be chopped to eliminate 80% of it. The 6-speed is really spongy you have to watch shifting from 5th to 6th as it wants to jump left between gears. The stick on mine has a bend in it about 5 inches down. I have not attempted to remove the center console as yet to see whats underneath. Soon though soon. Getting the radio out of the dash was a 90 minute job but not to bad. Perhaps the dremel and I will hookup this weekend.

Ekh.

The 350z has the exact same transmission, so maybe you can switch out the shifters.

It's not the transmission it's the shift lever length. Cut the dammed thing off its long so it acts like a lever. You probably can swap out the shift lever from a Z but why go to the trouble.
When in doubt garb the Dremel.

Ekh.