It is relatively easy to renew your passport from the US. The biggest hassles are a) a guarantor who meets the new criteria of being a Canadian citizen passport holder who knows you more than 2 years and b ) getting the proper size and quality of passport photographs done in the US.
For the first, you have 3 options: you can use the old form which allows you to use an American guarantor who satisfies one of the professional categories and knows you more than 2 years, or you can use the new form and use a Canadian citizen passport holder who knows you more than 2 years, or you can get a special form "In lieu of a guarantor" and have a notary public or Commissioner of Oaths sign it if you don't have anyone in the above 2 categories.
For the second you will really have to shop around. The Walmart or pharmacies quickie passport photo option won't work as Canadian passport photographs are a different size than the American ones and Passport Canada is much more stringent on having no shadows, not smiling, the head being a specific size within the photo, proper exposure, etc. A professional camera store or a professional photographer can do the job - just provide them with a copy of the passport specifications provided by Passport Canada.
Since you are sending off your original birth certificate and unexpired passport with the new passport application - you can request the return of the old one which will be canceled - send it by courier to the Passport Office in Hull, Quebec. If everything is fine, they should process it within 10 days and courier it back to you. The majority of rejected applications are because the photographs are not good enough so really do make sure it is good enough, not just 'close', as it will cost you time and money to redo them and re-submit them.
Here is the relevant link:
http://www.ppt.gc.ca/cdn/form.aspx?lang=eng®ion=USA