Jerk in the transmission or diff?

In my 91 5 speed i can be going down the road at highway speed (55-65) in 5 gear and once in a while i feel this jerk like the truck comes out of gear for a second but then goes back fine,no loud noise. Just like you took it out of gear then put it back. I only happens prob once a week but the truck shifts fine, anyone have any ideas? Is it the clutch, throwout bearing, transmission, rear diff??