gotcha. weird. have you checked the level to see if it's over or under filled?
simply check fluids and whichever is loosing fluid is what is leaking. check engine, trans, front diff oil and see which one is low.
if it's blowing diff oil out the top - it may be leaking down and caused the dealer to misdiagnose the leaking diff below.
it usually only blows diff oil out the top if it's over filled and/or running hot (ambient temps/highway speeds)
from what you said i'd be skeptical of the dealer. the only differential seal you can replace for external differential oil leaks is a one hour job and the seal and oring costs $20 (there are two, one on each side). $1,600 is shocking.
it's easily visible, right where the axle connects to the differential/transmission. the seal is right behind there - it's either wet underneath the axle or it is not. takes about 1 minute to diagnose properly.
you could even simply take pictures. here is what it looks like with the axle removed, look for that ring - it's very close to the front diff oil drain plug - take a picture of drivers and passengers side by that toothed ring. but you should be able to tell if it's leaking or not.
if you're uncertain clean off all the old diff oil up top and below, check levels, and check it again later.
be sure to ask first before forking out that $1,600.
for $800 i'll come do it for you.
you can see in the picture above the 1 bolt required to replace the seal once the axle is off: remove that bolt, turn that side diff retaining ring and the seal is right there. that ring does set the backlash for the front diff and your trans is hosed if you dont' do it right - but that simply means you need to be able to count to do it right - mark it, count turns to remove, reinstall to the exact same number or turns and mark.
if you frequently use that dealer i'd be cautious about their quotes and diagnosis.