I use the IRS pinned diff setup in my Associated RC10L2 pro ten car. It works perfectly. I drilled 2.5mm holes in the axle hub and diff hub, and press-fitted the yellow wire into it. Make sure the yellow wire sits flush with or lower than the diff ring, otherwise your diff will bind.
It works just as good as the D-drive setup, but you don't need to buy other hubs or axles to make it work, just drill the holes!
If you need replacement diff rings order IRS511N. It includes 8diff rings, and some yellow wire.
Before you toss a diff ring sand it down on fine-gritt sandpaper, till the groove disappears. You can use the same diff rings a long long time that way, without any ill-effect on your diff action.
As far as I know, the Rug Rat still is a top-of-the-line kit. It should be on a par with most other aftermarket conversion kits, like those from IRS, speedmerchant and Dpowell.
Fatdoggy, have a look at these parts: IRS4014, on this page:
It's the usual Associated dynamic strut front arms, only machined to sit lower, so you can run your tires down to the rims.