Is this a death rattle for my T5? (I'm afraid so)

I've had this problem with the T5 in my Cobra clone ever since I've had it, but I've been paying attention to it recently. There's a rattle/vibration in the tranny in low-speed/high torque scenarios. For example, while driving up a parking structure, I'm driving slow in 2nd gear. Taking the turns, I usually coast; when I hit the gas after the turn, I get a rattle for a couple of seconds. If I give it just a bit of gas around that turn, no rattle. I also hear it when shifting from neutral to 1st.

I was under the car this weekend, and decided to check it out, My first thought was the rear end, but I was wrong. With the tranny in 1st, I can rotate the driveshaft by hand about 1/2"; at each end of the rotation, I hear an audible "clack" in the middle of the T5 somewhere. I did not notice any side-to-side movement of the driveshaft yoke nor any play in the differential or the aluminum driveshaft. No other trouble shifting into gear that I have noticed. Some whine in 3rd gear, but that is improving since I put in Royal Purple Synchromax this weekend (which is why I was under the Cobra)

So, the rattle is caused by play in the tranny somewhere, which means a rebuild. Any other ideas? The clutch and t/o bearing are likely OEM Ford; the donor Cobra only had 52K on the clock. Oh, in case it's relevant - I have noticed a slight vibration starting just over 2K RPM's, and it goes up and down with RPM's. It's only a light vibration, easy to miss if the road's a bit rough. The balancer was replaced when I did the driveline swap, a Pioneer 50oz SFI balancer. I'm not sure if the vibration is new or if it's been there the whole time.