I appreciate the trust -- incidentally bestowed upon someone who is not an
in the trade at all. What I know about auto repair comes from tinkering with
my own vehicles over the years -- somewhat assisted by a large pile of books
and a couple engineering degrees (neither in the automotive field). As my
own '98 Expy has the 5.0L engine and the 4R70W transmission, no direct
experience here. Take what I say with a large grain of salt. If you get
lucky and someone like Jim or Tom respond, follow their advice and ignore
what I say...

First, it's worth verifying that all those codes are systematic and not
something random. Not sure what you mean by 'diagnosed'. I am assuming that
they ran on-demand diagnostics, and not just retrieved stored codes. I will
also assume that the vehicle has no significant drivability issues, though
you did not say anything to this effect. So here is the best I can say about
your list of trouble codes:
1) Believe it or not, the most common reason for the 'gross evaporative
leak' code P0455 is a loose or poorly sealing gas cap. Take a look --  
nothing to loose there and you may get lucky.
2) You told us nothing about the history of your vehicle so this is a wild
guess. The 4.0L SOHC had a common problem with intake manifolds, resulting
in 'unmetered air' (vacuum leaks). Those could explain the 'lean' codes
P0171 and P0174. Unless you know that the problem has been taken care of,
it's worth checking. Google the archives of this group for details. This
stuff could also be as simple as a open vacuum hose somewhere or a faulty
fuel pressure regulator - hardly a big deal.
3) I will be on a very shaky ground even trying to say anything about your
transmission-related codes. Assuming that yours is the 5R55E, my book says
that P1715 indicates a 'mechanical' shift solenoid failure. P1762 could be
either a solenoid or a band failure. It should result in losing 2nd or 5th
gear - something that you should have noticed.

I can't really answer your question about dumping the vehicle. It's a matter
of philosophy more than anything else -- I keep a 22 year old LTD with 200K
and it still passes emissions with flying colors. My philosophy is that if
the body is sound, the rest is worth repairing. As I do most of the work
myself (and enjoy it), this philosophy may not suit you.
With the possible exception of P1762, which could (or could not) be a band
failure, I don't see anything here that would require a major teardown of
either the engine or the transmission, though the bill could easily reach
$1000 and beyond - depending on the shop and what exactly they find. Yet
it's an 11 year old vehicle approaching the end of life for its major
components, especially if they have not been meticulously maintained; and
you may be faced with bigger expenses in the not-so-distant future. The
choice is yours. If you decide to go for it and don't have oodles of money
to spend, stay away from the dealer. For the engine-related issues find an
independent that has not been asleep for the last 20 years, understands
OBDII and has a decent scanner - not too much to ask for. I can't believe
that California cares about your transmission selecting gears properly or
not. If they do, find an independent transmission shop that your friends
were happy with. Not every transmission shop is a scam operation. I once had
a problem with erratic shifting that produced no codes. The owner of a small
shop told me that he was too busy to work on my vehicle for the next week,
but "get a good look at the EPC solenoid yourself" - and it was right on.
You know who gets my business next time...

Good Luck!