I know Currie make some great joints, but the plastic or rubber inside of them can wear and loosen, hopefully not after less then a year. I'm not sure if the Johnny joint you have, have the internal snap ring holding the joint together. I had a rear track bar from Currie and the snap ring popped out of it grove and gave me a fare amount of movement. As fare as I know, you are at the very limit on pinion angle of the drive shaft, it might not take much movement to cause vibrations.
Other then that, I'm sure what else it could be with out seeing and hearing the jeep.
If you do try loading the drive line as I described before, make sure you do it in forward and in reverse in 2wd.