GM-GMC: 1983 GMC serria 2500, gmc serria, parking brake cable

Question
My 1983 gmc serria 2500 has a few problomes most of which i thank are rear end probloms.  when i get to about 35-40mph. my truck vibrates and stops a few mph more. my rear tires (both of then) wear toword the inside. my left rear seal (where the 8 bolts are) leaks.  maby you could help me out. if u thank that its the rear end then tell me how to put a different rear end in it.  if i can stop the tire wear and the vibration then i can work on the rest of the truck please help if u can.

                 e-mail:bmx_man_415@yahoo.com
       
                        Thanks
                        Britten

Answer
Your tire wear problem is very unusual.
First, make certain that the wheels are true and that the tires are not out of round or have other defects (tires are far from perfect, in most cases).  Also confirm that the tire balance is correct.
How are the shock absorbers?  If worn out, replace them.
The leak at the seal indicates a worn out outer axle bearing and both outer bearings and seals (on both sides) should be changed.
Lastly, a driveshaft imbalance or U-joint can cause the vibration you are experiencing.
If you can find a good used rear axle in a salvage yard, with the same axle ratio and from the same type of vehicle, it may be more economical to remove and replace the entire rear axle than to repair your old one.  Replacing, and some service work on, the axle requires disconnecting the brake hydraulic lines and/or the parking brake cable.  Be very careful to restore the brakes to proper operation after performing any work on the rear axle.