and if its not been changed in the last 20K miles then drain it off and fill with fresh. I can strongly recommend the use of Castrol Syntrans 75W90 (was called SMX-S) as I've known it help with shift and noise issues on many manual gearboxes
and yes greasing the prop joints and centre bearing is also well worth while also
you can easily check for for any slack in the propshaft joints by twisting either half by hand and feeling for any play as there should be virtually none
the centre bearing is quite loosely mounted so this can be moved normally but if you can run the car in gear with the wheels off the ground and then feel the centre bearing housing for vibration if there's any then consider replacing the unit
and then look along the propshaft for any signs of a missing balance weight as these take the form of little 3/4" squares that are welded onto the shaft - as they are only tacked on they can sometimes come loose and be lost resulting in the shaft being out of balance - if a joint is worn this can also shake them loose
so if a joint is replaced its good practice to have the shaft re-balanced afterwards
The manual gearbox itself seems perfect. The previous owner was a lifetime manual driver and drove his kids around in it, so I think the gearbox got treated comparatively gently. Thanks for the advice on the center bearing. The noise is actually worst just following a harder acceleration, so that might just be the fact that the shaft is under greater torque and distorts the placement a bit more given the loose mounting/old bearing and takes a moment for the vibration to get absorbed. I'm sure the bearing isn't particularly expensive, so if/when I get it serviced, I might just have them replace that regardless.I've read about the the best ways to change the TC and diff oil, do you have any particularly clever way you go about it?