Driveline Hitting underside of floor (at rear)

I just read the moparts tech article. Interesting. The first section "setting pinion angle" appears to me to be for everyday driver type cars. It makes sense for production cars,not high performance modified cars. The second section is taken word for word and picture for picture from the mopar performance chassis book. Its interesting that a picture there shows a driveshaft angle being measured. Third section seems to draw information from both previous sections and as such is rather confusing. For myself its obvious. I use the performance chassis book as the base for mods. Of course its highly biased towards drag racing but then so am I.