Hopefully Jim will give you some advice, but i suspect your car is running a front diff (same as in the back) with the one way on the shaft, I may be totaly wrong. Do you have a manual, page 2 of Jimmy's review here:
http://www.oople.com/rc/photos/zx5/zx5review02.html
shows the diff arrangement (no bearings on the shaft in picture) and a one way arrangement (with the bearings).
Not a lot of detail on the shaft, but at the front end before it meets the front diff there is a one way bearing, you can remove this and there were parts in the original kit to make it "solid".
Guess if you are sitting twiddling the wheels, you should be able to see where the one way starts & finishes, when we ran oneway in the front diff Jordan tore off a wishbone and we lost the outdrive, that just sits in the bearing, i am having trouble discribing this, if you pull the link off twix top of front hub & bulkhead, the wheel & wishbone will swing down, driveshaft will come out of the outdrive, if the outdrive then easily slides out, its part of a oneway, if it dont its a diff.
Jordan drives all the time with two diffs & one way on the shaft, we won't change it. hint
I think there is a manual in the review somewhere, try searching on here if you havent got one.
Here:
http://www.oople.com/rc/manuals/zx5/
What tires were you running, sure I saw it with spikes, try yellow pins
Edit: Page 8 of the manual = Diffs (third option as well) part 89 is the outdrive, ours is still in the grass at Silverstone, page 9 the slipper, page 17 the shaft. Should keep you busy
Mick