I actually made this account specifically to reply to this post. Are you still having this problem or did you get it fixed? I purchased my 06' XA with 128k and it also had this issue, which was quite annoying because the car sits at 3500rpm @ 70mph, my normal highway speed. Like you this happened in all gears, regardless of speed, beginning about 3300 and ending about 3800 - and only when the car is in gear.
So just FYI these are the repairs I have made to date:
New rear axle and axle seals
New brakes and rotors
New wheels and tires
New control arms
Front end alignment
New belts (both)
Clean throttle body
2 full cans of Seafoam run through fuel system during several fuel-ups (premium fuel)
New Accel coil packs
Replace airbox with Injen CAI and TBS
Replace intake manifold with Weapon R model
Replace exhaust header with Weapon R model
New stage 2 clutch from Clutchmasters
New light flywheel from Clutchmasters
Theres more but I am losing track... the last part with the clutch kit and flywheel was specifically done to address this 3500rpm vibe issue, the thought being my pilot bushing might be shot - clutch needed to be done anyway. I completed that job today and the test drive seems to indicate some of the vibration went away, but I will need more time to tell. The next step is replacing the exhaust from the header back with 2.5" pipe, a high flow cat, and a Flowmaster DBX muffler to get some better flow - hopefully smoothing things out further and fully utilizing the header/manifold/CAI/TBS combo.
I hope this helps if you are still having the issue, I will post back in a few days after I've had time to test the new clutch out.