Having some issues trying to get my car sorted out.

Recently killed 2nd in my old box. This now broken box did about 3400 rpm @ 100kmh. I believed this box to be a 4.11, and had a replacement 4.11 put in. This new box is running at about 3000 rpm @ 100kmh. So the most likely conclusion is that the old box isnt a 4.11. The rear diff wasnt replaced, so now im looking to replace the diff to match.

My questions for the gurus are:

1) What ratio is my old box and rear diff likely to be?
2) If I replace the rear diff, will i need to replace the rear drive shafts also?
(I believe this may only be the case if i get a non-GC8 rear diff)

Anyhows, i just wanted to make sure I had this sorted before i went and replaced the rear diff. Any help is much appreciated.

Cheers,
Smokey