When to Change shocks on a 1987 GMC Sierra Classic truck.?

[badassnova72@att.net]

I bought a 1987 GMC Sierra Classic Truck back a year ago, all I know is that the truck sat for probably over 2-3 years never moved sat in one spot. I had to rebuild the engine, exhaust, change transmission fluid and differential fluid all the basic routine maintenance. Before we even started working on the truck we found some papers that showed the last oil change and basic maintenance the date was back in July of 1996. And i dont know if any shocks have been replaced or if they are the same shocks that were on the truck since 1987. So should i go ahead and replace the front and rear shocks or how long do shocks usually last. The man has since passed away so i cant ask him. Also at the time that i bought the truck there was only 58000 original miles on it.