FAG Rear Wheel Bearing Seal Kit Without ABS

What are the main areas to look out for on my Toyota 4x4 leaking rear axle?
OK, whilst a fairly basic setup, oil leaking into the rear drums is one of the most common issues that seems to occur.
If we think about the logic behind the setup then we'll try to help explain what may be occurring - or at least things to look out for - of course, any info is given in good faith, after all, we are parts suppliers and not the mechanic!

So oil coming from the diff into the drums can only pass by:

1. The Inner seal leaking between seal and axle casing - is there damage between the outer casing of the seal and the axle casting it fits into - has someone previously been heavy handed and worked a screwdriver between and so potentially allowing oil to pass?
2. The Inner seal leaking against the retaining collar - The same seal has an inner lip with compression spring behind that runs on a highly machined hardened collar, is the seal running true to the collar and on the flat face of the collar - has the spring become dis-logged when the half shaft was previously installed or is the seal worn or damaged - always lubricate the lip of the seal with oil prior to fitting the shaft.
3. Is the collar leaking where it meets the half shaft - The collar that the rear wheel bearing is designed to be an interference fit onto the half shaft, this collar not only acts as a secondary backup to the bearing snap ring, but is also the machined face that the seal runs on.
   Should there be any marks or damage to the half shaft that the collar sits on, then again, oil could potentially run between collar and half-shaft.
   Unfortunately many times we have heard of these collars previously being cut off with grinders or Oxy-acetylene leaving marks on the half shaft - oil can then run along even the finest of marks.
   This is why Toyota suggest using the bearing puller so as not to mark the shaft when removing.
   
   So this is how oil can pass the seal or collar - However, other factors can cause this passage of oil;
4. The axle breather - A blocked breather on top the casing could be blocked, pressurising the system forcing oil past the weakest point - usually the oil seal, try removing, cleaning or replacing the breather, takes seconds but is often over-looked.
5. The rear wheel bearing - A worn rear wheel bearing can in effect cause the half shaft to sit off centre to the seal - again causing oil to leak between seal and collar - replace the bearing with a new one or a kit.
6. The half shaft - At worst the half shaft could be warped - unlikely but is a point that Toyota make in their manuals is to check the axle shaft run-out Unfortunately this isn't a part we supply.
   
   So hopefully this will allow you (even your mechanic) to gain some information in order to inspect and check areas of potential faults that can, and do occur in this setup.
   Remember, good mechanics should always look for a cause and not simply the obvious, always look for the potential reason / cause prior to replacing any part