Yeah I do mean a double cardan. I'm having one built right now off of the joint from a front driveshaft off an 85. A bit cheaper than just getting a totally custom build. Next step will be to see if i need to notch the horsecollar to get it to fit, but a lot of people don't seem to need to do this with the 85 4runners.
I wish I had recorded the sound while it was happening. Definitely a distinct chatter from floating the throttle (it would happen between shifts at higher speeds) and another similar chatter that would happen when decelerating (worse in neutral or with clutch in). Hopefully one noise from the sloppy u-joint (the one at the t-case flange) and then the other noise from the slip yoke play. Noise was very random and chattery if that makes sense, not like a typical steady vibration. Almost like a loose exhaust part.
The plan here is to have a 4runner with 35s and dual cases that actually likes doing 75 on the freeway. The chatter was enough to keep me from going over 60..