the spring mod on the propshaft is a method af ensuring the propshaft is pushed all the way back into the rear pinion, as it was discovered that maybe the rear pinions were breaking because the propshaft was moving too far forwards into the front gearbox under certain conditions.
The spring allows the propshaft to move forwards and backwards as necessary when/if the chassis flexes without anything being damaged, and is probably more useful on a propshaft that has not been built precisely.
The question about needing one or not is entirely up to you.
The bearing mod probably related to the propshaft being supported by two bearings instead of one - again this is not a necessary modification. The X10 transmission, if built correctly, is bullet proof "out of the box" now.
As for diff/thrust balls, I have no idea - there is a thread on
www.the-pred.co.uk that discusses the use of Tamiya thrustraces etc., but having never had an issue with the ceramic diffballs in the pred, i wouln't know which others fit the car.
Hope this helps.