84 Spider Transmission question

[Paul J Weimer <pjweimer@domain.elided>]

I've had this 2.0 Spider for about three years.  It has been neglected
(parked in the barn) the entire time.  Started working on it a couple of
weeks back, lots to do.  After replacing the dash, I attempted to start
it to see what work may be needed under the hood.  No start.  No spark. 
After much diagnostics, I found that the rev sensor was problem. 
Borrowed one from the 88 and she fired right up.  Not bad, I thought,
after three years.

As I was backing it out of the barn to put it on the lift, it kicked out
of reverse.  Now the transmission was stuck in gear.  Seemed to be in
5th, just a guess.  No matter where the shifter was, neutral, first,
second, etc., when I let the clutch out a bit it was in a forward gear. 
I was trying everything, but always it remained in this same forward
gear.  At one point a rather violently jammed it into 5th, pulled the
shifter back to neutral, slowly let out the clutch and things were back
to normal.  I again shifted to reverse and she backed up just fine. 
Pulled it onto the lift, no problem.

A day or two latter, after attending to such things as replacing 2 feet
of oxygen sensor lead, repairing the battery wire insulation that had
been stripped of under the car, etc., I took it out for a short drive. 
Again, as I backed it off the lift, it jumped out of reverse and there it
was, stuck in a forward gear once more.  This time I immediately repeated
the process that gave solution before and it worked first try.  Drove it
about 8 miles and she ran like a top and shifted without incident.

I figure this tranny must come out have some extent of surgery.  Was
wondering if you all might give me a hint on possible causes.  I do all
the work that I can on my Alfas, but have yet to need to crack a tranny. 
Am I looking at extensive and expensive surgery?

Thanks in advance for ideas and suggestions,
Paul Weimer
69 Boattail
This 84 2.0
88 Spider Veloce
91 164L

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