One thing you can try is to get the rear of the jeep on jack stands, put a dial indicator on the slip yoke, turn the driveshaft and measure the runout of the whole rear driveline assembly. If I remember correctly by Spicer standards it shouldn't have more then .008. If it's more then you can start isolating what the cause is. Yoke, SYE, driveline, Etc. Then for example say the total run out of assembly is .014. The driveline is .006 and the transfer case tail shaft/yoke or pinion/yoke is .008 you can sometimes rotate the driveshaft 180 degrees and get the two differences to cancel each other out so your total is .002. I kept rotating at the pinion, the transfer case, and the slip yoke, measuring and marking each time until I was able to get it down to .005 on my jeep. Almost no vibs now.