Red,

Is the fast clicking sound tied to engine speed or to wheel speed? If tied to wheel speed, it might be something to do with the drive shaft especially since you described a vibration/noise at 3 equally spaced speed intervals (imbalanced rotating things tend to have different vibration modes at multiples of natural frequency). 

Also of note, I had a similar fast clicking sound during deceleration but not during acceleration (but mine was present no matter what speed i was going). It turned out that my driveshaft u-joint was contacting a bent heat shield. Basically under load (acceleration), the torque forces your pinion down toward the ground, pulling the driveshaft away from whatever it may be hitting. During coasting, it relaxes and pinion rotates up toward the bottom of the car, bringing the driveshaft closer to whatever sheetmetal is above it. But again, would be a weird coincidence if this was the case.

It could also be a flat needle bearing in your u-joint. The reason u-joints need a bit of angle when installed is because without this angle, the needle would never rotate in the bearing cups on the u-joint -basically they would cease to be a bearing. Under acceleration, when everything moves around under load, the goal is zero angle at the u-joints as this provides for the most efficient power transfer. Consequently, the needle bearings are not rotating during this brief period.  So if it is a wiped needle, no rotation means no ticking noise (under acceleration) and rotation means a tick every time the flat portion of the needle rotates past the load bearing surface. 

So my thought would be to check the driveshaft, u-joints, and things causing clearance issues first (bad rubber mount, bent sheet metal, etc). If everything looks good then maybe it is the rear end. And again this all assumes it is a wheel speed tick, not an engine speed tick. 

Just my $0.02

One other thing to check when light grinding noises are apparent, is for a sticky wheel cylinder. Might be a dragging brake shoe - they can make light grinding noises - but that generally manifests itself with grinding and making the car pull left or right which you haven't mentioned.

You sound like someone who is fairly on top of things, but i like to check the easy things first. Side story - had a friend in high school with a mystery ticking noise coming from the back of his car. He parked the car and bought a rebuild kit for his rear end, got under the car and found a zip tie around his driveshaft. Never bothered to check anything before buying parts, HA! We haven't let him forget about it either...

But, if you are hoping something is wrong so you HAVE to get a new rear end, I'd just mentally convince yourself that the grinding is a pinion bearing and the mysterious ticking is a chipped/cracked pinion tooth...."but honey, it could explode an leave us stranded in the middle of nowhere! I have to fix it!" -honey could refer to a wife or wallet... your choice...