It should be easy to make a stock height K5 run smooth. Maybe go back over the basics:
-Is the U-joint really seated right in the yoke? (front and rear).
-Is there any sign of missing weights from the driveshaft?
-Are the U-joints good?
-With the driveshaft off, is the yoke nice and tight?
-What did that rear axle come from? Maybe the pads are at a different angle?
-Is the slip yoke tight in the transfer case output?
-Is the transmission mount tight?
-How was the front regeared? Was the whole front end swapped or just a new ring and pinion put in up there?
-Anything else loose in the back like U-bolts or broken center pin?