I'm no battery expert, but from my research it looks like AGM batteries require about 14.4-14.8V to charge properly. That's a lot more than the 4Runner alternator can provide.
Some folks have used modules to bump the alternator output to the higher voltage. The one downside to that is that some components in the truck (e.g. light bulbs) may not be designed to operate at the higher voltage.
Other folks follow the strategy of using a charger to occasionally charge the battery fully, since chargers can provide higher voltage. That seems reasonable, but is one more thing to keep in mind.
Yet others have said that they've ran the AGM battery as is, with no issues. Though the counterargument seems to be that by doing that, the life of the battery will be shortened significantly.
So, it's not as simple as it seems

. I'd still love to get an AGM for my next battery, just want to make sure I know what I'm signing up for.