Oil coming out of the prop shaft connection at the read extension means the seal is worn out. Early gearboxes only will have had a felt seal here, and the later rubber seal can be adapted to fit. The felt seals will always leak a little.
Clonk on moving off can be a worn universal joint in the prop shaft but this will also give vibration at speed. Look for rusty dust inside the universal joint and check for any play. More often than not it will be a rear damper sticking or wear in the diff. It can be sticking rear brake cylinders in their backplates, especially if you just reversed, the drove forwards. Live with the diff wear, if it is reasonably quiet in use?
If you have just had the driveshaft off, and hear a clunk, check that all four bolts are TIGHT.