My experience with engines starting to shake at idle was from a valve starting to burn causing low compression in one cylinder.

Might do a compression test to make sure you don't have a weak cylinder for whatever reason.

And since you just adjusted the valves, might recheck the clearances.. Too little clearance on a valve on one cylinder?

2000 4Runner,V6,Auto,31's, Sliders, Skids, On-Board Air, Air Shocks
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