You only need the shims if your going to change to 4gear ... All the parts to change to mid motor are in th box, as for the 4gear change aslong as u buy shims of the right size you don't need the durango ones
but try mb models cpd racing and inside line racing seem to have bits I had to use a combination of the 3 to get all the parts
failing that bud if it's just the shims you need and can't find any of right size any brand iv got a cpl ye can have if you send ye addy iv also got the idler gear pin too as bought two lots cos they ws the hrs thing to get hold of altho I suspect a diff brand would work again aslong as right size