Is it a good idea to purchase the install kit with a ring and pinion set? OR will the shop have all the seals?

Im looking at the motive 3.90.

thanks,

 

That's going to depend on the shop you take it to. If they do lots of diffs they may have all the shims you need but its unlikely they will have the bearings and seals and may charge a fair amount more then you can buy them yourself.