Does it 1) force you to hold the wheel in one direction, fighting to pull the steering wheel the other direction? Or 2) does the steering wheel just become off-center and if you hold it straight the 4Runner does not travel straight?
People have described both of these conditions as a pull, when a pull is actually only the first condition.
Mine recently was doing the second condition, not fighting, but steering wheel off center when in 4 hi. It was an outer tie-rod going bad and had alot of play in it.
If yours is actually the 1st condition, if you have access I would put it on a lift and run it in 4 hi, comparing the wheel speed of the two front wheels. This is a weird one, if something was dragging, you'd think it would be dragging when the front wheels were not pulling too. Even without a lift, get the front wheels off the ground and compare spinning one vs. the other, that's where I would start to track this down.