Terry, I've had about the same experience as Jerry with our '06 except the failure occurred at about 12,000 miles, at which point I had one of the Bullet Proof copper/steel units from Mac's in Portland installed. That lasted exactly one week after the two year warranty expired before it began leaking from the rear tank seam. It was rebuilt locally here in San Diego and so far, fingers tightly crossed, appears to be doing well. Apparently the radiator problems are the nature of the beast.
Jerry, your bolt together unit certainly sounds interesting; although the bolt together charge air cooler a lot of us have, and I had to have rebuilt with new gaskets certainly didn't guaranty to be failure-proof.
The $5,500 figure for the R&R of the coolant radiator is definitely in the ball park. As in most of these replacements, just figure the price of the new unit and add that again in labor, or even a bit more; that seems to be the norm these days.
Not to get your hopes up, but if you haven't already checked the upper radiator hose clamps you might just get lucky. Last fall I too saw the dreaded slow drip of coolant and figured it was the radiator again, fortunately it was just a couple clamps that needed to be snugged up a bit.
2006 Alpine 36' FDTS