Rear Suspension Coil Springs problems of the 2002 Ford E-350

This 350 econoline, 8cyl. , extended body van was purchased new, and I am the original owner. The vehicle has only 120k miles on it since 2002. Beginning several years ago, my mechanic complained of severe frame erosion, having had to reinforce and re-weld several parts back onto the frame, including but not limited to the brackets which support the rear leaf springs. (two ocurrances-- left and right sides). Last week, my employee complained of some rear end swaying at moderate speed cornering. We discovered a cross-tie connection between the side rails at the rear of the vehicle's frame had rotted away, thus causing some unwanted frame movement. If it weren't for a heavy-duty towing package I had purchased with the vehicle in 2002, I believe the left and right sides of the vehicle's under-framing would have twisted and separated from the body years ago ! also beginning a few years ago, was the decay of the vehicle's engine block, as three separately timed spark plugs had literately 'blown' out of the threaded wells of the engine block. My mechanic had to adapt heli-coil kits into re-bored holes in the vehicle's spark plug wells. It's only a matter of time before these kits come loose, and blow -out again. I'm already experiencing a painfully ominous compression loss in one (or more) of the cylinders. Could Ford have purchased some inferior metals, thus producing a line of junk vehicles?.