Measuring torque

That's how I do it with my fishing scales hooked over a bar of known length.

Tightened to, or with some other component tightened hard until the pinion shaft needs this torque to turn it ?

If it really is this fastener torque (small carb components are about the only common one) then get a torque screwdriver. They're expensive, but keep an eye on eBay.

If it's the usual diff pinion problem, then the suggested way is to wrap string round a shaft of measured diameter and pull it with a spring balance. You then need to _read_ the balance to know what the current torque is, when what you really need is a preset at the right torque.

This is a total pain in the arse to do if you're working on your own, so an easier way is a bar across the propshaft flange boltholes and a known weight hanging off the end. A sockful of bolts and the kitchen scales will make a calibrated weight for you. An assistant is still useful to lift the bar and re-drop it as needed.