If it came with the "green stuff", then you have to put the "green stuff" back in. If it came with the "orange stuff", then you put that in.
I'm sure a '98 is the same as a '97, and it has the green stuff. Just used the off the shelf Prestone. You can also check the spec's on their web page. The spec for your truck will be lised under the hood on the rad cover.
Prestone had the same spec for their stuff. I'm going to be doing the second flush on mine before winter.
If you are planning on storing yours, I'd flush it now. 50k is a lot of miles for the original coolent. Besides the miles, it's over four years old. Should not go over two years at most on coolent. It'll get 'acidic', even if it looks good and says it still has good temp readings and then you'll be replacing the heater core really soon...... JMO, but that's been my experience with the stuff.