Started hearing and feeling a "thunk/clunk" sound from the rear end a couple weeks ago. I recognized the sound as the slip yoke sticking after hard braking. Thought it was odd though since I had just greased all the u-joints and rear slip yoke just 4000 miles ago during the last oil change. Seems like it should have lasted a little longer.
Anyways, got in there and using Gamefreakgc's trick of sticking a little jewelers screwdriver into the zerk nipple —I found there was no grease under pressure at all. Last time I greased it, I greased it until the driveshaft started to extend.
Started pumping and managed to get 47 pumps in before the driveshaft moved again. I then let some grease back out the zerk with the jewelers screwdriver.
I'm not sure where all the grease went in just 4 thousand miles, but I can say that the clunking noise is gone again and there are less vibrations at 70mph.
I wonder if the higher temperatures (106 the other day) have thinned out the grease and it just kinda gets used up quicker. I didn't see any sign of it being spun out all over the place, so who knows.
Oh well, just par for the course I guess when you drive a nearly 20 year old 4wd with 375,000 miles on it. Grease is cheap...