Actually, he did specify 4x4. So, you're saying it's going to cost him
$4,000 to re-gear?

Geez...  where do some of these numbers come from?  A rear R&P set is $300
($225 for a cheap set, but let's not cheap out here).  $100 for an install
kit.  $20 in gear oil.  On the few axles I've worked on, it took me an
average of 4 hours to do the job.  I'm slower than a pro who does this
daily, but let's stick with my numbers.  Even at dealership-type labor rates
of $100/hr., that's still $820 - call it $900 - for the rear axle swap.

On the front, an R&P set is $250 (again, could have it for $190, but that's
Standard Gear vs. Yukon - I think Yukon's worth the extra $$), another $100
for an install, and call it another $20 in gear oil.  The front diff's more
of a PITA to work on, so call this one 8 hours of labor.  $1,200 for the
front.

So, we've got about $800 in parts, 12 hours of labor - throw in another $50
for shop supplies and whatnot, and I figure just over $2,000.

Now...  how do you get to $4,000 again?