How many miles have you done in your Shox? Most runners change pairs every 300-500 miles (depending on your weight and how heavily you land). New aches and pains can be an indicator that it's time to get a new pair (though they could just be due to your increased mileage and pace - it's a tough one to work out)
The main reason to get checked at a shop is to determine how your feet roll through the gait cycle. Most people roll too far in (overpronate) and so some shoes have stiffer cushioning on the inside to try to reduce this. Some don't pronate enough, and need as much cushioning as possible.
I don't know much about the Shox, but you don't see many "real" runners wearing them - initial reports seemed to suggest they weren't that successful, and were very heavy. They may have improved since I read those though.
If you go to a proper running shop (not a rubbishy general sport shop like JJB - they dusually don't have a clue and don't often stock many proper running shoes anyway) they will advise you as to what type of shoe to get. Usually they'll get you to run over a special mat, or watch you on a treadmill. They can tell a lot from the wear pattern on your existing shoes. If you don't want to go to a shop, you can get a good idea by doing the wet footprint test described somewhere on this website. You can save quite a bit by buying last season's models on the internet, though obviously it's a bit of a risk with getting the fit right. (The show review section on this website is invaluable in working out which shoe falls into which category).
