If you're on good terms with the Apaches -- I mean, the Centaurs ;-) -- of the Thundering Steppes, this one should be an easy solo, so long as you look sharp for wolves. The Centaurs won't attack you unless you attack them first.
If you're on bad terms with the Centaurs (i.e. you've been killing them) you have two choices: either put together a group and prepare to fight your way in and out, or else repair your reputation with the Centaurs by A) not killing them any more (duh), and B) killing lots and lots of Gnolls. That's right, Centaurs hate Gnolls as much as you do. I don't remember the exact ratio, but it's something like, for every 3 Gnolls you slay, your faction standing with the Centaurs goes up a point.
Let's presume you're friendly with the Centaurs. Either walk, ride, or griffon fly to Thundermist Village. Go north out of the Village until you're on the main road. Follow the main road west until the cliffs on your left turn south (you can see this on your in-game map if you've already explored here). Follow the cliff-line south (watch for nasties on your exposed right side) until you start to see Centaurs ahead.
As you approach the Centaurs, you'll see a canyon on your left, cutting through the cliffs. Enter this canyon and follow it.
At some point, you'll get a game message that you're in The Shattered Gully. This means you're in the right place. Centaurs good. Wolves bad. Keep an eye out for wolves.
Eventually, you'll come into an area that looks like it's filled with broken trees -- all the tree trunks are short and bare, and splintered at the top where the rest of the tree should be. You're close now.
There's one bare tree that's noticeably taller than the ones around it, and which isn't broken and splintered. Approach it, and you'll get your acorn!
Last thing: if you continue following the Shattered Gully, you'll cross a wooden bridge and come out very close the SouthEast Griffon Tower. This passage -- which doesn't show on your game map -- can save you a lot of hiking through hostile territory when travelling eastish-westish in the Thundering Steppes -- particularly if you haven't done (or don't want to do) the rather pain-in-the-**** Bridge Keep Quests to gain access to that passage, further north.
Of course, you've gotta be on good terms with the Centaurs! ;-)