Hi BP - I feel your pain. My OS driveshaft seal leaked on my old Ti 'box, and when that was scrapped off (becuase the speedo gear drive was rusted into the casing and couldn't be removed), the replacement Gold Cloverleaf 'box leaked as well
Sudboy is right, replacing the seal is a complete pain, as the halfshaft has to be removed from the bearing carrier (the bit with the square flange that bolts onto the side of the gearbox). This is held in place by a threaded bush that is pressed onto the shaft. A special tool and a lot of force are required to remove it. Alex Jupe stripped down my old gearbox and I inherited all the bits after it was replaced - inspecting both the seals (the one that was oil-tight and the one that leaked) revealed absolutely no difference between the two - no nicks, no obvious sign of wear etc.
Fortunately, before Alex scrapped my Ti 'box he sourced a replacement bearing carrier from Just Suds to fix the original that had one of the cast alloy flanges snap. So I had a spare OS bearing carrier to try - third time lucky!
I fitted the bearing carrier last weekend, and the one that came out had noticeable play in the outer halfshaft bearing (rocking motion in the halfshaft). So I think perhaps that play in the halfshaft was compromising the seal - it would also explain why the oil contamination only occurred when on the move - at rest the seal was fine!
My replacement does not exhibit any play in the halfshaft bearing, and so far (one brief trip of a couple of miles at speeds up to 50 mph), there appears to be no leaks...
You are lucky having the NS seal go, as you don't have to remove the speedo drive to take it out. Driveshaft off, brake disc off (you are inboard, aren't you?), undo the 4 Allen bolts holding the halfshaft carrier in place, and withdraw the whole assembly (don't forget to drain the oil or catch what comes out in something). There is an inner bearing which stays on the differential cluster - the bearing race is pressed into the halfshaft carrier. I took advice and fitted my replacement without the compatible bearing race - in other words, the inner bearing was as before, but the halfshaft carrier race was for a different bearing. My engineer friend said this would be OK as long as the race was not excessively worn or damaged, which it wasn't. To do the job properly you would need to press the complementary bearing race into the carrier, and to do that you would need to remove the halfshaft retaining bush etc. etc.
Good luck sorting your leak. I will let you know whether my repair works out...