Ok, so although I have a 3rd gen, I have been posting here since this is where the info all seems to be.
As I said above, I have ordered the All-Pro slug adapters (BEST CUSTOMER SERVICE award goes to them BTW) - they measure exactly like the OEM balljoints so that should be good.
I also ordered 25mm rod end seals from seals-it and I will test fit those (about $16 from eBay)
While looking at the different uniball control arms, I noticed that ICON has a completely different design for their balljoint adapters and hardware. Instead of using a pressed in slug adapter, the whole thing is designed so that the bolt comes up from underneath and threads into the lower spacer, the top spacer is then bolted into the lower spacer - thus clamping the uniball.
It looks like a lot more thought went into that setup and I wonder if they will sell you just the hardware?
Take a look -
58400 - 96 - 02 toyota 4runner tubular uniball upper control arm system
Study the pics a little and you will see it is a lot more ingenious.
ALSO - they do also make a cap to cover the uniball (actually how I found their site) But it is too low profile to fit on our uniballs.
What I did think of is that it would not be hard to find a lid that will fit, a cut down spray-can lid would work if you glued it on with some RTV - not nearly as elegant as a CNC billet anodized piece - but would get the job done, I'm sure with a little more searching we could find a solution.