A clutch at the very earliest stage of being worn out will behave like this. When you’re starting out or making a shift, the clutch is already moving relative to the flywheel, so it only has to overcome the kinetic friction in order to slip. Whereas if you stomp on the gas in 5th on the highway, it’s stationary relative to the flywheel and therefore needs to overcome the static friction, which is harder to do. Given time, it will start slipping in gear.
I guess a hydraulic problem is possible, but IME hydraulic problems that stop the clutch from engaging are fairly rare and when they do occur usually affect the clutch release as well, and the OP didn’t say anything about having trouble getting into gear as well. I will send kind thoughts that it might be a $30 hose instead of a $1000 clutch, but I wouldn’t hold out too much hope.
Unless you are an exceptionally patient and/or cheap person, you will get sick of it slipping all the time long before it actually gets so bad you can’t get it to move and it strands you somewhere.
When the time comes, definitely do shop this around. Some shops can do it a LOT faster than the book calls for, and so this is a job where there are big price discrepancies. Also ask how much replacing an automatic on your same truck would cost, which will make you feel better about your clutch repair and owning a manual in general.