checking rear end housing straightness

excellent info. Actually I have an 8&3/4 and a 5:57 pumpkin (probably deeper than I need) with welded spiders but the dr diff axles will not go in all the way. still haven't figured out what is wrong there.
Had the same problem with Dr. Diff axles, took a triangle file to splines to remove any burrs, then they slid right in.