Is it time to bid farewell??

I am at that cross-roads where I have to make a big decision. My 1997 F250 is still running strong, but I am concerned about the body and chassis rust. I bought the truck new and it has spent it's entire life in upstate NY with tons of road salt. My primary concern is about chassis integrity and degradation of towing capacity. I intend to keep using the truck for conventional towing of loads upto 12,000 lbs. I do not see any holes in the frame yet, but maybe I haven't looked in the right places.

Questions:
1. How concerned should I be about the chassis rust?
2. How do I check and evaluate the rust?
3. Are there any known trouble spots to look?
4. Has anybody seen a frame or hitch failure?

I am in the position to upgrade to a newer truck and plan to look at used 2008's on Monday, but I am still not sure that I need to spend all of that money to remain safe.