And just to add to what Kyle said, the non-TRD rear diff has an 8.25" (sometimes erroneously stated as 8.4") ring gear. The TRD rear diff and the older, pre-Tacoma 4x4 rear diffs are 8". As for the spline count, I have no idea.
I have a lockright, but have no experience with the no slip. The difference between the two is that the no slip is supposed to have better street manners. Full-time, automatic lockers like that the lockright lock up both axles with the application of torque. So if you give it too much gas while turning a sharp corner, the tires will scrub with a lockright. Supposedly, the no slip helps lessen or prevent this from happening altogether. The other difference is that the lockright is less expensive than the no slip.
I remember reading in the old, delphi forum about some who've broken no slips, but I don't remember reading about anyone breaking a lockright unless they were running much bigger tires (38"?). However, the argument is that no slips break because of faulty install - whether that's true or not, I don't know. Hopefully someone with a no slip will chime in.
Hope this helps.