KeithJ. The original design has two bushes, 67 3073, the one innermost to the gearbox is drilled to match oilways in the final drive sleeve gear. Just as bergs explained. From my experience original bushes are relieved where they join to form a small oil reservoir. I have seen boxes where these two bushes have been replaced with a single bush, or pattern bushes with no relief butted together but this seems a bad idea as there is nowhere for oil to accumulate to aid lubrication.
Effectively sealing the shaft end with the SRM nut certainly stops the drips, but also prevents any through flow of lubricant, which is actually desirable (if messy) on a design with concentric shafts running at different relative speeds in the intermediate gears. On a tight shaft this may be just enough to give a seizure. Something has changed if the box has covered some 5000 miles without problems. More common to tighten up in the first couple of miles with an assembly problem.
With the shaft seized, mainshaft and sleeve gear rotate as one, the normal mode when in top gear. Third gear introduces a change in relative speeds, but not as much as in second or first gear. So a partial intermittent seizure could still give you third gear.
With the clutch pulled in pushing with the box in top gear (mainshaft and sleeve locked as normal or seized) would be possible. Probably not in any other gear with a true seizure.
Swarfy.
Additional. Re reading added posts, an internal fault due to inadequate lubrication, or an external problem, as suggested (chain tension) are good places to start looking for a solution.
