1986 Chevy Suburban 2WD 454 w/ 3 speed AT 

Consistent transmission fluid leak (maybe 1-2oz/week), replacing trans
pan gasket (and filter) didn't help at all. When cold, the truck takes
approx 30-60 seconds AFTER shifting to DRIVE or REVERSE before it will
move - but otherwise functions normally - no unusual trans sounds,
trans fluid full/clear, good power, normal shifts, etc. Could this be
a torque converter problem - the leak and the slow-to-engage issues?
Question: how can I isolate the problem (torque converter or
transmission) prior to replacment?

For what it's worth, my family was having a similar problem with a
1991 Ford Aerostar minivan.  When they took it to the transmission
shop, it was determined that it had something to do with the pump that
circulates the fluid (I'm sorry I don't remember what it is called)
and the fact that one of the gears was just gone (I think it was 3rd
gear, which they said would also affect reverse).  Once thing that we
did was just made sure that we had transmission fluid with the van at
all times.  When we went to have it fixed, the guy said that it
wouldn't be worth the cost of repairing compared to the condition of
the car.  He also said that it may work for years with that condition,
but maybe only days.  Of course, this was the van that was driven back
and forth to work and that was it, so it wasn't in the best of
condition.  It got so bad that it wouldn't go into reverse on a cold
start at all, so we would have to back into every driveway and parking
space because it would go (eventually) into reverse while it was warm.
 I'm not a mechanic (obviously) but I've been there.  Just my cent and
a half.