Ron,
Point the rear output flange straight to the transfer case and the double cardan joint will be vibration free, otherwise it'll vibrate worse than a conventionally jointed driveshaft when the output flanges of both the diff and TC are off more than 2 degrees from parallel.
I'm betting you've already covered the bases and comments about were tongue in cheek, but if not, may check the output flanges of, at least the rear to make sure the rear flange is at 0 degree to drive shaft angle, or 2 degree variance that's stated acceptable.
Can't fathom why the front would need anything beyond single joints, but surely a reason.
The front shouldn't vibrate at speeds typical 4WD operation, but wouldn't think a standard would either.
A rear with a double cardan that isn't within the parameters above will vibrate like crazy, at least in my experience, being the lifted 80, which is the most sensitive on the planet, since AWD.