1999 ford f250 zf6

I have a 99 ford f250 I bought a month ago. The other day I went to take off in 3rd from a dead stop and it popped out of gear when I released the clutch tried again and same result, put it in 1st took off no problem shifting through the gears no problem upshifting down shifting no problem. Any ideas?

Worn synchro won't let the gear fully engage from a dead stop.

Is this something I should stop driving it for.

No, just be a bit gentle with it, keep an eye on the fluid, etc.
They are rebuildable, or remains are easy to find.