Front drive shaft longitudinal play

I'm hoping to figure out what's causing a significant rubbing sound when 4wd is engaged on my 2003 santana this weekend. The sound is coming from the front diff. When I have the front jacked up and spin one wheel and allow the other to spin the noise is not present, however when the other wheel is held, or I turn the front drive shaft directly, the noise appears. There's also quite a bit of forward/backward play in the drive shaft (the u-joint appears fine).
I've changed the oil recently and it seemed ok - stinky but no nuts or bearings in it!
I'm thinking it's time to open up the diff and see what's going on inside. Am I a fool for thinking I can remove the drive shaft, the bolts circled in red, and the gasket gunk to get enough access to inspect things?

Having looked at the picture I would suggest that the pinion seal is wearing probably because the bearings are dying. Taking the bolts out will just let the oil out but not allow inspection. You will have to remove the half shafts.

Thanks Lambert! That makes sense. Sounds like it could be quite an involved job with crush spacers and preload... reckon I'm in for a 'fun' learning experience!

The pinion seal was indeed rotted away, as well as all four bearings. The sensible option would have been to get a second had diff but I had much more fun* rebuilding this one. I was astounded to eventually finish it and find that it didn't explode on the first outing. And now I know all about backlash, preload, tooth alignment, and spending too much money!

* cursing, bleeding, hammering my thumb, breaking and reordering bearings, putting together and measuring and taking apart several times

It's a good feeling when you succeed with something you don't normally tackle. A used replacement is the quick fix, but then you don't know it's condition and you don't learn anything.
As a new diff expert - would you do it again balancing cost / time against a used item?
Just interested in your experience, luckily no need to do anything this complex at the moment.

Well done!

Mike

That's a great question. With the knowledge and tools I have now, I think I probably would do it again myself. For someone who hasn't done it before though it's not worth doing for a cost or time saving alone. I believe a second hand diff can be got for around £75, and I've spent north of £150 on bearings, seals, and tools. Even without the tools you'd be hard pressed to get all four bearings, oil seal, and crush spacer for less that £75.

Assuming the gears themselves are good though, you do end up with a diff in good condition which, as you point out, a second-hand one might not be. Although it's possible to check the condition without disassembly by measuring the backlash, preload, and tooth alignment with the right tools (dial gauge, fish hanging scale, and engineers paint).

Another factor to consider would be safety - I felt far more comfortable working on the front diff that I would have on the rear. If I got something wrong then it's a case of disengaging 4wd and heading home. If it was the rear diff the consequences would involve a van with at best a flashing yellow light.