I am really intrigued by this problem. As mentioned several times earlier in this thread, the transmission was designed before WS fluid was the norm. When ours started acting up, I figured I would do a drain and refill and being the cheap-o that I am, went to Wal Mart and purchased Wal mart Type VI fluid, which is allegedly compatible with the WS fluid. I drained the pan, put 4 qts back in and leveled it off using the jumper wire, pull the plug technique. Since then, we have had none of the vibrations.
Did the Type VI fluid somehow change the properties (viscosity, temperature-viscosity relationship, compressibility, etc.) just enough to mask the problem? Was swapping out 20% of the fluid enough to remove some crud that was causing problems? Did the problem just go away on its own? Was our transmission low on fluid before and the addition just brought it up to the correct level?
In any case, I submit my experience as simply that. If it was not cost prohibitive, it would be cool to get a 4Runner and remove all the WS fluid and replace with Type IV (the heavier, previous generation that the trans was originally designed to use) and see what happens. Something tells me that it might just work...
Another idea that I had, when looking at the almost watery consistency of the used WS fluid was the old friction modifier trick. Perhaps the fluid still meets viscosity requirements, but its lubricity has been reduced. Again, I did not do this, just spit-balling here.
