Reference Greg's points about driveshaft vibrations, you might want to look at the Driveshaft Shop site below. It has a tech section on setting driveshaft angles as well as a "vibration" section that includes a couple of techniques for isolating vibrations. I had them build my half shafts and driveshaft (3.5 inch dia/24 inch long). I had the driveshaft balanced for an 8000 rpm critical rpm (actually 7926 rpm -- 6500 engine rpm divided by .82 overdrive) and the half shafts for 2600 rpm (8000 rpm driveshaft speed divided by a 3.08 gear ratio). I also balanced the stub axles and the posi carrier (with ring gear, bolts, and safety wire). My earlier angularity discussion was in reference to lateral, not vertical alignment of the crank and pinion. There are a lot of opinions about the vertical alignment, mostly dependant on the pinion wrap-up or pinion angle change induced by suspension geometry changes during travel. The objective is to get proper geometry when the suspension is "loaded" -- easy for us independant rear suspension guys, since nothing moves. Also note as Greg pointed out, that while the pinion angle does not change significantly with a 4-link, IF the links are parallel and equal length, but the pinion angle will change during suspension travel if, for example the upper links are much shorter than the lower links, as is often the case. I assume that since you have experimented with a wide range of pinion angles, you have eliminated that as a vibration issue.
http://www.driveshaftshop.com/driveshaf-menu.php