make sure the e brake cables are fully released and just because the pedal is all the way dont mean the released. If thats not it pull the diff cover off and see whats going on in it. If it a looks good at least it has a fluid change and its simpler to pull the cover than swap rear ends.

 

I hear ya I tried retracting the parking brake cable but that wasn't it but he already had his new rearend put in but im going to buy his old one and figure it out.

 

Damn It !!!!! I read the title "thinking rear end", and so I naturally started thinking non-truck topic and got excited ..... oh well, never mind .

 

Well if you have an IPhone the have a mustang forum just like this.