understanding tire load rating

well I made my first long haul with my new to me 5ver, my 5ver did well being used I was very happy with it, how it pulled and maneuvered. that's why we buy 5vers isn't it. I did however have tire problem with my truck, (I should have known better) the truck was given to me and had some front end shimmy which went away when I rotated the tires. on my return leg off a 1000 mile trip I picked up a bad shimmy/vibration (slow you down to a crawl vibration) found a bad tire driver rear and mounted a spare, (this was the one I figure might have caused the slight vibration when in front) 100 miles up the road picked up another vibration and the other rear was going out)
so I hobbled back 10 miles to the nearest open store last night (Wal-Mart) put two new tires on the rear. tire guy told me the tires that came off were from 02 and probably went bad due to the load on that age of tire. Comments ? also with a tire rated at 80 psi max cold what do you recommend or prefer to pull at ?? I was set at 70 psi cold before I left on the trip.