TR4 - Rear diff. When to rebuild?

[Christian_Simonsen@ena-east.ericsson.se (Christian Simonsen)]

     I'm an electrical engineer who dares dabble in the mechanical way of 
     the world. It was too worn to test run so I opted to complete a mostly 
     cosmetic rebuild of my uncles TR4. (Also, Rebuilt suspension, steering, 
     brakes - you know, essentials)
     
     Now, it runs well, but..... 

     
     I thinks that the diff on me solid axle TR4 in on its way out. 
     U-joints are tight but I get a clunk when lifting/pressing the 
     accelerator pedal with a heavy foot.  
     
     Jacked it up and I can spin one wheel back about 15 degrees (? 
     probably more) before the other wheel starts to spin the other way.
     
     Is this bad?
     
     How bad? Is it just an adjustment?
     
     When should I take it off the road?