I didn't even consider the diffs, good call. In my opinion grease doesn't semi-lock a diff. It may provide enough tension to wheely off the line but if it can't carry it at all it's unloading. If the diff unloads on water you're going to lose momentum. If you're serious about running on water I'd lock the center diff somehow, silly putty, 500k fluid or whatever. Semi locked would be using 50k fluid as it will still unload, but it won't unload as soon as the front tires are in the air.