I'll go for low ground clearence to stop rollovers on the high grip grass tracks because the stocktires really dig in, do you think that proline road tires would work on grass to give more predictable handling?
There's a good UK (where I live) company that sells T-Maxx hop ups and they have drive shafts for £25, that's $39 (without the expensive shipping)
They are universals, not cvds but look pretty strong.
Check them out at www.cmldistribution.co.uk/fast_trax.htm