I think you could explain this easily. The Empire had been intruding on the Ewok's moon for quite some time, so the Ewoks could have made the traps before the Rebels even arrived. It is not like the Ewoks would have just accepted the Empire's intrusion and not been the least bit afraid of them.

In a small/medium-sized country in the real world when there is unrest near a border, the local military gets beefed up because the likelihood of a conflict has been increased. If your country gets invaded then you would form a guerilla and you hide arms and make traps at various locations in case you would need them in the future.

You could find the Mythbusters episode on Youtube quite easily if you searched for it. Spoiler: The armoured truck gets smashed. You could surmise that the armour of the truck, like the AT-ST's, was made to withstand small arms fire - not a large pounding. The logs in the movie were also twice as long and pulled up higher.

In the Ewok movies, we also see them having beasts of burden and we can assume that they are skilled enough with ropes/vines and wood to make pulleys. Also, C-3PO's recognition of the Ewok language as being a dialect of a language known on other planets does indicate that Ewoks have come from another planet (moon) originally and/or would have had interaction with other worlds, which means that they could have imported knowledge for how to lift them up.
... and it is not like there aren't examples of great feats of engineering in real-world human pre-history.