Transmission Over Temp - 2000 Jeep Grand Cherokee Laredo

I recently purchased a 2000 model Jeep Grand Cherokee Laredo. It had
120,000k's (don't know what that is in miles!) and had full dealer
service history. I continued the servicing of the vehicle with timely
precision. I was driving along one day and the "O/D Off" light came up
on the dash and stayed on. I thought I must have accidently bumped the
overdrive button. I tried clicking it on and no matter how I clicked it
the "O/D Off" light stayed on the dash. Next the "trans over temp
light" came one. I thought "Oh Dear!" I drove a little further until I
could get to the nearest mechanic. By the time I arrived a funny
whurring sound was coming from the gear box.
The gear stick now 'slips' into gear and when driving the gears will
not move up or down. It gets "stuck" in gear (auto transmission
obviously) - the car went into "limp" mode (that's what the jeep guy
said) he cleared the codes and I can drive the vehicle but not very far
or very fast. It's been sitting in the driveway for 3 weeks now.
Of course everyone is an expert - but I would love to hear from anyone
who might have ANY idea at all as to what the problem here could be.
The best solution I have been offered is that "the gear box will HAVE
to come out regardless and nobody will know what is wrong with it until
that happens"
This I have been told will set me back anywhere up to $6,000.00.
What i want to know is why someone who is a "transmission expert"
cannot even give me a clue as to what the problem might be.
I await your replys!!!!!
Thanks in advance!
Lyndall