Sounds like wrists to me. I had the same problem which meant I always done front squats with the arms crossed/bodybuilder grip but a bout 6 months or so ago I managed to be able to do the olympic grip (without tearing my wrists to pieces!). I actually find it's easier to keep a good back position and not lean forward when using that grip.
What I done was drills out of Inside Out. I didn't make it a priority I just threw things into existing workouts. The one's I used the most was...
Get a bar racked and just try and get into the olympic grip front squat position, whilst it's still on the rack. So, you grab it, spin your elbows round until it get's a little uncomfortable, then return to starting position. The bar doesn't leave the rack, you're just spinning your elbows under it as far as you can go. I done this mostly 'dynamically' (8-10 reps) but occasionally held it for time, too. There's nothing to stop you doing this between sets when you're using the squat rack.
The other which 'feels' the most effective is to stand side on with a bench or step or anything (i sometimes do this using a desk at work). Place both hands flat on it, with the fingers pointing towards you, palms down. Leaning on the palms slighly, move you're arms/elbows backwards (sort of 'rock' back) until it feels uncomfortable and repeat for the desired reps (again I shoot for 8-10). This feels brutally effective. Sometimes I held it for time on right side because the right side was more restricted than the left. Again, this can be done between exercises or sets, it's very convenient. You can do it anywhere, on a desk, on the floor, etc
On top of that, i'm in the habbit of doing wrist stretches throughout the day now. There's wrist stretches that I use in the following article,
http://www.tmuscle.com/free_online_arti ... amp_wrists
I'm not too specific with what one's i use, i just try and stretch them out a few times a day when at work.