I am inclined to think RWENUTS has it nailed. The U joint is most likely just a symptom for an underlying problem .
The 4 link should maintain the angle pretty well, but if the pinion angle isnt correct to begin with, the U joint would suffer first. Ulimately the pinion bearing will fail.
I totally agree with checking the pinion angle. Kudos RWE.
Fact is stranger than Fiction