I would say might be driveshaft.. If its coming from whole car length... You can go underneath and check the ujoints... But really wont know till shaft is out and check the ujoints. .. Its easy job and some aftermarkets are like $300..strictly speaking 2wd 4runners.. Atleast in my car problem was the rear ujoint on the driveshaft.
But if yours is 4wd unit.. Then cost may be higher.
Check out my write up.. link....
Toyota driveshaft or rebuilt one locally?