Is this the rear U-Joint?

Basically, here's the deal.

I have no problems taking off from stopsigns, it's smooth.

But, if I stay in gear, above 4500~ or so RPMS, my shifter feels like it's turned into a vibrator. It gets worse the with higher RPM. I believe it's also making a slight rattling/clinking noise at higher RPMs as well.

Also, when I'm cruising on the highway at 70-80, this vibration happens regardless of the RPM. It's less severe at 3k rpm, but it can get worse.

Is this the rear U-joint? I know those are non-replacable, so I can either buy the Mazdatrix replacement driveshaft, or hope the guy selling the replacement U-joints gets my e-mail or PM.

 

Early last summer had the exact same problem. But got worse the more miles I drove the car. Vibration ebventually worked it way down to very slow speeds. I also suspected it was a U joint despite the absence of the "clunk" when shifting. Took it to a respected RX7 mechanic who confirmed the U joint failure. Put in a new drive shaft and all is well.

 

Awesome, thank you.

 

As a U-joint starts to fail, it first looses it's grease, then the needle bearings start to wear and make slack & excess clearance. The extra wear allows the shaft to wobble off center giving the vibration.
Then the needles start to rub & bind on each other.
As they bind & pop free from binding, you can hear a "tink" sound that rings the tubular shaft.
This can usually be heard just as you start moving.

 

The RX-7 sound dictionary:
Thunk (when starting out) - Front diff mount.
Tink (when starting out) - u-joints
Clunk or groan (with suspension movement) - old grommets.
Zoom zoom (short cycle) - vac leak
Zoom zoom (long cycle) - TPS or BAC or grounds
Crunch - high rev shift to 2nd
Beeeeeeee - over rev or low coolant or low oil.
RrRrRrRrR - starter gear sound (normal)
rrrrrrrrrrrr - starter gear sound (flooded) (no compression surges)
RrRrRrrrRrrrR - starter gear sound (bad apex seal) (uneven compression surges)