Sorry I can't offer too much on your rear axles issue, but if the speedo is that far out, I would think the transmission speedo cable gear is incorrect. There are various gears available, you just have to figure out which one you need, BUT your rear axle could also be NOT the ratio you think it is.
Years ago, I owned a Chrysler 5th Ave. It blew out the spider gears, so a mechanic buddy swopped in a junkyard rear axle. As soon as it was driven, the speedo would show like 70kph when really it was doing 40. Later I found out it was from a 6 cylinder car, not a V8.
LSD, do you mean a Traction-Loc? If so, you might want to go to a 3.50:1. Mine just got a Traction-Loc installed over winter, but I kept the original 3.25:1. It could definitely use a bit more grunt off the line, but still have good highway mileage.
Edit: Boilermaster beat me too it , but yes tires could also be in issue. His comments on shocks are also valid. You might want to upgrade your leaf springs to 4.5 or 5 leaf. If you do, BUY Eaton Detroit Springs springs, not Grab-A-Trak junk. (Ask me how I know!!)
