I'm going to agree with Barry's comment about "Spicer". Moogs quality has slipped a bit from what I've noticed. I do believe they have 2 lines also, good and better. Or in todays world, cheap and decent.
No way is the slightly off driveline angle causing the problem. LOTS (that's L.O.T.S.) of trucks, including mine, have much more of a driveline angle difference and don't bust joints like yours have been. At least not that fast. I replaced mine 20 years ago and they are still fine, AND they are the greaseable type with the zerk in the body AND suffer from severe axle twisting under a hard load, not having any bars on the rear. The fact that they were so hard to press in tells me that quality is a bit off. They shouldn't be that hard to press in, and I'm sure you've done this enough to notice the difference between snug and TIGHT. Being that tight is putting undue pressure on the caps, and they are very hard/brittle. With the top of the cap being solid and the bottom open, a really tight fit can slightly oval the open end. It may still turn easily, but the pressure will work its evil magic. I've also never been able to overtighten a strap using just a 1/2" wrench, which is what I always use.
I'd go with Spicer, and if they're still really tight, maybe run a brake hone in the yokes to enlarge them a bit. I'll bet your problems will disappear.