Can I build a diff myself, and anybody used this tool?

I have done many set up's over the years. Probably the most important thing is getting the Pinion depth correct. The tool you referenced from Summit may do the job. I have never used one. That being said, the correct tool to use is much more comprehensive that that one. The tool I'm talking about. mounts in the diff bores, and does a much better job. I will add, that if you are only changing pinion bearings, most of the time, you can reuse the original set up shims and be fine. I does sound, however that you are changing the set completely. In that case you will have to set the pinion depth from scratch. On most gear sets, the pinion depth is scribed on the nose of the pinion gear. You can try it initially with the original shims, but you may need to add or subtract from there to get it correct. That depth scribed, is from the centerline of the diff to the face of the pinion after installed. After that, you need to make sure that you set the preload on the pinion bearings. Then you can install the ring gear carrier and set side preload. As you set preload on the ring carrier you will have to check the ring - pinion clearance. Most are somewhere around .07 - .011. Double check that a few times and maybe run a test pattern. It is not as hard as people make it out to be, If you take your time and have the correct tools. Hope that helps.