I haven't done the Grampians trail but I've done plenty of others on all sides of Victoria and I can tell you, it's a gamble. The difficulty of some is overstated. Others are the opposite.
Even on what's regarded as an easier track the problem is it only takes one unexpected obstacle to stop you (or stick you). A bit of surprise rain etc. like albat already mentioned.
In Gembrook there is a track called Blue Range, it requires a proper 4WD with low range but is relatively easy and any stock 4WD would normally be able to get through it fine. I'd done it myself 3 or 4 times before at this point, and was out again with a friend who was taking his new truck out for the first time.
It was just after the track had opened again from seasonal closures and it had been raining heavily in days before. Sure enough, one of the hill sides sections which was normally a flat dirt road was now a 20 metre long, metre deep pit, with 15" deeps ruts full of mud the whole way along.
There was a bit of a queue with another convoy when we got there checking it out. I don't know the exact specs/mods of his vehicle, but the guy who went first and the only one to get through was in a short wheel base patrol with lockers, a massive lift and 37" (or thereabouts) mud tyres, and he made it out by a hairs breadth.
The rest of the vehicles were Landcruisers, D-Max, Patrols etc. Every single one got stuck and had to be snatched or winched out by the guy in front.
I think my last words before heading out with my mate were "It's pretty easy, you'll be fine"
Not trying to put your off your trip or anything, but it's a legitimate concern I think.