Craptacular8: Going from mtb to road bike can make a neck problem worse. The reach forward to the handlebars for brakes and gears will be greater, so you may find you are "craning" your neck more. This happened to me. In my case it was the result of weak deep neck flexor muscles that seem to be associated with a lot of computer use and the typical hunched forward posture. In many people i can be worked through, but can take time.

There are actually exercises online for strengthening deep neck flexor muscles... just google that phrase with the word cycling and you can find many cycling oriented sources that describe exercises.

On the other hand, that may not be why you get neck pain. For some people, it seems that being more stretched out on a bike is a better posture. I have not experienced this and cannot relate.

I would note that before buying a road bike, I cycled for quite a while on a mountain bike with 1.5" (40mm) road tires. I don't think the bike and gearing were holding me back on the road using the right tires, but if I had trail/off-road tires on it I would have been much slower. You may want to consider new tires as an interim measure, unless you find you have to spin the crank too fast when going fast on your current setup.

Whatever you do, don't buy a bike just because the owner is of a similar height/inseam to you. It is irrelevant what the seller's height or inseam are: the bike may not have fit the seller. There are online calculators that will tell you what size of bike to start with and probably the LBSs were pointing you in a particular direction as far as size. It is most important to buy a bike that is the right frame size and to test ride it first. After that you will probably need some help/advice to get the saddle height, saddle setback and stem length set up.