Vibration at ~2400rpm in 3rd (auto-tranny)

I've noticed a slight vibration (through the gas pedal) and sound change from the engine when my 2000 XTerra 4x4 V6 is accelerating between 2300 and 2450 rpm in what I think is 3rd gear (automatic).

Is this something I should be concerned about or is it normal?

So I can assume this is a unique problem that I should see my dealer about?

There have been some complaints about cracked exhaust manifolds. I haven't heard of any transmission problems.