Just over 2 years ago we bought a house with a dated A/C package unit.
14 months ago we replaced the A/C with a 3 ton Trane package. After
about 9 months of improved cooling over the old unit, we began to
experience problems with cooling, pipe leading from the condenser to
the coil froze, etc.

Dont understand why it could be working for 14 months fine, then a
return air duct mysteriously becomes sealed? You would have gone
thru a heating season in that time, how on earth did the heat work
during this time since it shares the same ducts?