H2 stock shocks life span?

[h2con]

I have a O3 with a little over 70K on her. I'm the original owner so it has not been abused. Just a little trail driving. What is the life span of the stock factory shocks? I was looking at some Skyjacker Nitro 8000's. They seem like a nice shock and there is a $35 rebate when you buy all four. That brings it to around $140 shipped for all corners. Is it hard to change them for a average shad tree mechanic? I'm going to tackle my rear airbags in the next couple weeks. They have decided to start sagging every once in a while.