It may be worth checking out eBay chop shops for a set of shocks for the newer LX1, LX1e, LX2, LX2e. I had your buggy and I know the lengths of the LX1 are very similar, LX2 shocks are same length as LX1 but with a larger bore. If you want to keep your shocks and try new springs look for LX1, LX1e springs, they'll fit. Sorry I don't have fluid weight recommendations.
As for the diff fluid weights, 7k/5k/1k, 5k/5k/3k or somewhere in that range are pretty common starting points. The common theme is to have the rear with a lighter weight. Since you're ballooning the front tires you may want to try upping the weight of the center diff, if you're at 5k I'd jump to 10k and you could always mix the two to bring the weight down to somewhere in the middle if you wanted to. But yeah, silicone fluid makes a huge difference over grease.