Oil on the clutch usually makes the clutch "chatter". This is also usually only when starting off from a stop. Once under way you no longer "slip" the clutch.
The "chatter" is pretty rapid also.
If excessive gear lash is causing this "bucking" then the ring and pinion won't last long. Jack up one rear wheel and with the truck in gear move the wheel back and forth to see if there is a lot of play in the rear axle. Or turn the drive shaft back and forth to see how much it turns before the wheel turns. Just make sure the truck is blocked so it can't roll. Make sure the truck is well blocked so it can't fall.
I describe "bucking" as acting like it has kangaroo gas in it.
