Yeah, like completely different actually. A secondary heat exchanger is just a baffle, or tubes, some way of lengthening the exhaust path, allowing the flue gas more contact time with an "exchange" surface area so as to give up more of its temperature to the air or water on the other side of said surface.
Secondary combustion is just that, a firebox designed to sustain burning the wood gasses and/or smoke resulting from the primary wood burning fire until the wood burns into (or at least close to) the charcoal phase, at which point there should be little to no smoke, so little to no creosote.
Any ole burner can get a kinda/sorta secondary burn during high firing, but true secondary combustion requires either preheated air brought into the top of the main firebox via tubes right under the baffle, or, preheated air fed into a "nozzle" that the flue gasses/smoke have to pass through on its way into a secondary burn chamber, and then on out through a secondary heat exchanger(s).
The Tundra and Caddy furnaces are the "tube" type burners, most all of the newer EPA style stoves are too (except the ones with cats). All of the units that I know of that use the "nozzle" method are "gassifying" boilers, but there may be others, I dunno.
Then there is the Kuuma Vaporfire furnaces, I'm not sure where to classify those...maybe a hybrid? And there could be other types of secondary burn systems out there that I don't know about too...
It's not the money...heck, even the wife would be on board with a switch out because the Yukon is a lil complicated to run the way I have it set up right now. But it is running pretty clean and efficient and it would be a ton of work to change over. I had to completely disassemble it to get 'er in the furnace room. Also, I would have to install a new fossil furnace too since the Yukon is a wood/oil combo. I'd go with a HE propane furnace to free up my only flue for the wood heater...and my duct work is kinda complicated too. Oh, and I'd hafta install a 6" liner in my 8" liner 
