I know when I had my aftershock, I drilled out the top pivot ball and tower for 4mm bolts. I think they used plastic/nylon pivot ball stand offs. On the bottom though, they were captured, so the 3mm bolt worked ok.
You might want to try RC Raven's dual rate springs vs the strange setup you have...
I won a kit for that... sadly, still in it's bag. Right when the kit arrived, I went through my spare parts bin so I could make room and to remind myself what I had. Turns out, I have 4 brand new plastic big bore shocks just sitting in their bag waiting for the ones on the truck to die.
Since...
Losi calls them "Foam things"... lol
http://www.losi.com/Products/Features.aspx?ProdID=LOSA6003
http://www.nitrohouse.com/Foam-Things-Linkage-RingsAll_p_1357.html
I remembered having them on my xxx-nt 15+ years ago when I bought it new. Just dug through the online manual for it to find them...