truck shakes in third gear

[CecilDunn]

start off by saying i have this 62' c20 from the farm today i pulled out of the gas station and when i did i thought i heard something but didn't pay it no attention due to all the commotion at store once i pulled of and down the road shifted into 3rd gear and it started shaking horribly thought it might have just been bad onramp to highway i shifted into 4th and went on my way and it quit later down the road noticed it again shaking horribly once again in 3rd and goes away once ur above 2000 rpm in 4th never feel it in 2nd still new to the old farm truck and need to learn what this could be going to check plugs and spark to make sure its not a misfire idk what to do really any help would b greatly appreciated