I just replaced my 3.62 diff with a 3.38 sierra diff, the new drive shafts I have got are larger dia at the plug in point but the shafts are the same dia as the original 1600 drive shafts. What sierra would these come from - I thought only the 1600's had small dia shafts?

I'm afraid I had exactly the same propblem - except I didn't have the shafts to fit the 3.38, I just went back to the 3.62 diff! I'd love to know the answer too.
My 3.62 and shafts came from a 1989 2.0 Sapphire - even though the VIN plate suggested it had a 3.36 ratio diff. Ah well.