Just installed new clutch & replacement gearbox into a 96 wrx

When you approach approx 4k in third the rear of the car starts to vibrate.

It doesnt happen in any other gear, and only from 4k.

If you gently accelerate in third through 4k it's fine, it only happens when you gun it.

I've replaced the rear diff & prop, but the vibration is still there.

Has anyone else had this? Any ideas what it can be?

Theres plenty of oil in the box, which is supposed to have been checked over before being supplied, fresh oil in the rear diff, and the prop is fine on it's original car.

Mike