Glen, can't comment on the Rhino, but I installed a Walker 21468, (4"is 21471) a 3" straight through truck muff about 2500 miles ago and am really pleased. I'ed been straight piped, and while it was OK, I decided I wanted a little help in the 2000 to 2500 RPM range. I've done a fairly extensive sound mitigation project and my truck already had less interior noise than most stockers.
The Walker is just right. Still has some of that good big truck sound, about midway between stock and no muff at all.
This is the muffler that Dr. Joseph Donnelly was touting about a year ago.