It depends on the tranny you have, what year is the truck?
If you have the 4L80E in a 90's model truck you should be ok, if you have the newer Alison, which I don't think you do, you will be fine. If you have the 4L60E (the old 700R4) than I might be a little concerned. If the truck is older than that, say the TH400, you should also be fine. If it is constantly hunting for the OD then yes run in Direct, but if the engine has plenty of power (not dogging) in OD than most likely you will be ok, just watch the tranny temps, that is really what kills the tranny.
Give us a few more specifics, and we can help better.
It is a 1998. Not sure what transmission it has but can find out. There is no owners manual cause the guy I got it from lost it and doesn't know. Just wondering, it has 110,000 miles on it and just want to be carefull.
If you truck has the 8 lug wheels, then you have the 4L80-E. If it has the 6 lug wheels, you have the 4L60-E. In that body style, Chevy had two 3/4 ton models. The 8 lug was rated at 8,600 GVW, and the 6 lug was rated at 7,200. In my book, the 6 lug was pretty much a heavy half truck. If i were you, I would run in Drive if I was going through town at speeds below 55 or so. That way you wouldn't be lugging the drivetrain. I would also run Drive if you were on hilly terrain, which would cause the tranny to hunt between Drive and Overdrive. If you are out on good, flat open road, I'd say the Overdrive would be ok to use. My truck is a 5 speed, and I run 4th gear most of the time to make it easier on the tranny and engine. Hope this helps.