Usually this issue is caused with improper setting on the dash or the distribution baffle under the dash. Sure the radiator or heater core could need flushing or a change of coolant might be a good ideal, but that would be because of no heat. Sounds like a Dealer taking advantage of you to me. Find another Dealer or even a repair shop to get a second opinion!
Also it appears they want you to buy another vehicle from them and steal your Nitro that you think needs major repairs. With only 45K miles your Nitro would be considered having low miles.
Hope you don't follow their suggestions and find another source to just have your heater work on the drivers side. I had the same issue when my Nitro R/T was new and it was covered under warranty, and it was with the distribution door/duct-work under the dash near the floor. They ordered all new parts and installed to correct a know factory issue at the time.
Hope this helps out and please let us know how it works out for you, and Good Luck!
Nitro Year: 2007 (1 of 91,815 sold in 07)
Nitro Model: R/T 4X4 Stone White
CAT-BACK Exhaust, CAI, Projector Head Lamps
Fully-Equipped w/all factory options