You need t learn the "sex move" to crawl over stuff. Pop your front wheel up on said log, as a BMXer you probably can do a better wheelie than most, and then thrust your hips forward as if you are..well...you know...and then pull the bike back under you with your feet and arms so your body momentum gets the back end of the bike up on the log instead of cranking your wheel and spinning out when you lose traction. Can be done at whatever speed your balance will allow from a full stop between steps to just shy of bunny hop speed.