The problem with the hole in the diff being closed when bolted down, is that it must be bolted down to fill with oil... Which you can't do, since the case needs to be split in order to fill, but I think the vearings will leak the oil out anyway, so I will stick with grease or now until a suitable (cheap!) diff comes along...

The shocks are especially weak in the rear of the car. Backing off the travel limiting screws will help a lot (screws on the bottom of the arms, through the chassis) and give a higher ride height.
The different fluids will control the damping effect. I changed mine out to a 40 weight, r/c car damper oil. It's cheap for a good size bottle.

For taking jumps, look for a stronger spring, but be careful, as a stronger spring will put more stress on the weak plastic threads of the damper assembly. A replacement damper assembly and spring made from aluminum will be much better, and I think someone mentioned the car it would fit from, a while back in the posts.

Mark