If you're buying joints from a parts store, that's why you keep replacing them. The tolerances on parts store joints are not very close and cause lots of failure issues. I worked at a driveline shop for 8 years. I've seen too many junky "new" parts house ujoints fail within a couple hundred miles. Autozone is the worst. Find true spicer non greasable ujoints and problem solved. You'll spend 30-60 bucks per ujoint but they'll last damn near forever.
Truck: 05 GMC CCSB SLT Z71,
Go Faster: 5.3, 219/227 .607/.614 112, milled heads, eBay CAI, Circle D 2800, 3030 Built 60E, E-Fans, 3030 Tuned, LT's, Spintech Muffler, DuraGrip w/ 3.73's
Look Better: 10K HID's lows & fogs, tinted Chevy tails