A friend of mine has located a '83 sj 410 being parted out and tells me the diffs. will work in my '87 samurai and will lower the ratio from 3.73 down to 4.10. I can't find a cross reference or what ratio comes in these. My friend has good intentions but I need more info.  HELP!  I would love for this to be true because I want just a little more gear than I now have.  The reason is that I have gone to     235 / 75 / 15's  and still want to drive it no the street 90% of the time and would like a little more bottom (?) end torque without going too far.

Nope!

You also can't swap the diff gears over.  And the axles don't swap.  They are 3" to short.  Depending on the year your drive shaft will though. 

The transfer case does have the lower gears in it, 10% lower then the stock SJ-413/Samurai in both high and low range.  Just drop that in and it will bring you right back into stock speck with the 235/75 tires. 

Keep your old transfer case though, as you will want it to swap after market gear sets into.

Does that mean that it is lower all the time(2 wheel drive) or just when I put it in 4 wheel drive?         Also is it a direct bolt in or do I need to "Modify" anything.   Don't want to sound stupid but I don't know till I ask!

It is lower in both, the SJ-410 has the same high range gear as the 4:1 kits, but a slightly lower low range gear then the stock Samurai transfer case.

It should be a directo bolt in, they share the flanges until the 88.5 Samurai's.  Sometimes you will get weird ones though, so it is good to double check the flanges.  But the mounts are all the same.  I have an SJ-410 and run a Samurai transfer case, the SJ-410 one swapped into a Samurai to run the same tires you are looking at.

The only real issue is the E-Brake.  The SJ-410 has a drum style brake on the back.  You can pull this off and just put your rear flange on it instead.  I would hold onto the drum brake though, or sell it to someone who wants to put a rear disk brake conversion on.   They bolt right up to the Samurai case.