on one of my old cars I had the same kind of problem, it was a 74 camaro, it would eat up U joints like a Fat girl with a free bag of Cheese puffs. turned out that my rear end yolk was a bit loose, under load it would not make a sound, when I went down a hill it would grind like a Mofo and eat the Ujoint up. although yours is in the uphill area, mine was downhill. but as the above post states, check , or even just replace the ujoint. and inspect the rear yolk, make sure that the bolt is tight and torqued properly,. worst case scenario, the rear is out of gear oil and dry as a bone..