GW,
This is a slow and very deliberate decision you need to make. Before you make it you should have some good measures of the actual performance of your portfolio---"about 8 percent return" isn't adequate.
If you have enjoyed an 8 percent annual return over that period, you're doing nicely for an all equity portfolio. The return on the Vanguard Total Market Index fund was an annualized 4.28 percent from January 1, 2005 through March 31.
That said, you need to examine this in terms that have long term meaning. What is the asset allocation of your portfolio? What degree of risk are you willing to take, e.g. how much of a loss can you tolerate? One way to get acqainted with all that is to have your portfolio evaluated by AssetBuilder. It's a free service, no obligation, and it will help you start to answer those questions. We'd like it, of course, if you became an AssetBuilder client but there are many alternatives.
One is to build your own Couch Potato portfolio. If that's too complicated, another path is to invest in one or more of the low cost balanced funds that have excellent long-term records such as Vanguard Wellington, Dodge and Cox Balanced, or Fidelity Puritan.
In terms of financial assets and your age, I'd say you were in good shape and should limit your risk. Another good thing to do is to start tracking your expected Social Security payments. It should replace about 24 percent of your income, given your salary, if you can work to full retirement age. Your wife would probably take a reduce benefit at age 62 that would replace another 9 percent of your final salary--- so you'll replace about 35 percent just with Social Security. The remainder, which doesn't have to be anywhere near 100 percent, will have to come from investment income.
Scott