You might want to take a look at this website. http://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml

I think that you have posted thinking that you were getting some movement "axle wrap" of the rear pinion and that was causing your problem, etc. This might help explain it better and possibly give you an area to at least look at. I am wondering if the stock springs have weakened (saggy rear end syndrome) and is helping to give a little more vibration, changing the angle of the driveshaft slightly so that the angle with the rear pinion is not correct under load. I know that I am not explaining it properly.

I have a similar problem (vibration) at about 60-65 MPH that I cannot get rid of. I am doing one thing at a time to hopefully find the problem and I have done most of the stuff that you have already done here with no fix, yet. My next move is to replace the wheels and tires and see if it goes away. I think that my 2 rear wheels are bent slightly and give me that at speed. I am not having the problems you are having with wheel hop, yet.

Good luck!