This may be of use to someone....
The spline count relates to the differential end of the driveshafts.
Most of the older defender axles (pre-300TDI as far as I am aware) are 10 spline.
Some diesel defenders do have HD (Heavy Duty) axles, and as fas as I am aware all v8 defenders have HD (Heavy Duty) axles, these axles have 24 spline differentials + halfshafts.
The differentials (again as far as I am aware) on the 10 spline axles are called "2 pin" differentials (they actually have a single pin, with two planet gears) these differentials seem to be classified as quite weak.
The differentials on the 24 spline axles are called "4 pin) differentials (they have two pins in a cross formation, with four planet gears) therefore being a stronger differential.
The 24 spline axles (drive shafts) are also stronger (can handle more torque) because the splines are finer and are able to spread the load being exerted on them (surface area because of the pressure angle provided by each spline).
One thing to watch is that newer axles tend to have a smaller driving member (thinner), I am not aware of any issues, but in my opinion this would lead to smaller surface area being in contact between the drive shafts & driving members and would also mean that over time the thinner driving member halfshafts will show more wear (this may be a reason why some vehicles have driveshafts with integral driving members).
One advantage of being lucky enough to have a HD axle is that it can be converted to disk brakes (no you do not have to have a salisbury axle) but because the stub axles are the same stub axles as the salisbury drum brake axle, you can use the well known caliper conversion to convert these axles to disks.
The use of 24 spline axles is all a question of why you need to change them??
If you are going through diffs, then it may be a good idea + look at diff pinning at the same time or a limited slip differential (LSD).
If you want disk brakes then again it may be a quick + cheap route, note you may need to change the brake bias valve (PCV = Proportional Control Valve) as this would mean that the vehicle brake dynamics will be very similar to what LR did with the newer vehicles. I think you will need to source a PCV / bias valve off a similar vehicle (i.e. off a 110 or 90 if that is the vehicle you are putting the axle on) because the braking weights are different. If you leave the drum brake PCV then I think your braking will be affected.
Please note, all of the above may just be my ramblings.... do as you see fit, and I am sure that I may have got something wrong in the above explanation.
