Ohh the dreaded diff problem....
As David said Nathan (NTS) made the mistake of not bolting the diff up correctly.
There seems to be many options as to how to mount this correctly, just check out the Locost USA forums for a starter.
Even the main kit car 7 suppliers have their unique way of installing the diff so there is no one hard rule.
I believe the main concern is containing the torque on the nose of the diff.
Have a look at all the options (check out Keith Tanners problem with this) and see what you can come up with?
I believe Skov is the nearest to getting his car on the road and seems to have the nose of the diff under control with rubber mounts.
Some ppl go for the solid mount option but this may lead to fatigue problems in the future....
Just my observations but I'm sure there are more people out there more knowledgeable than me.