Been there. done that and its ugly!
My '96 rear main went out at ~23K miles.
It took 5 trips into the 5* dealer before
he admitted that it was a rear main.
Another 2 trips to fix it (or so they
said).
The straw came when I did an oil change
3K miles later and found that most (if
not all) of the bell housing bolts were
loose.

I mean the kind that you could
spin a couple of turns by your fingers.
Gee they seem surprized when I brought my
'98 back and told them all the things they
left loose after supposely replacing the
trans input shaft seal. Which they had
to pull the trans again 2 weeks later
after it started leaking again.
Now that it is out of the 3year 36K mile
warrenty I plan to do self maintainence
with a few excepts. Those I plan on taking
to Cummins West.