For me, vibrations only at a certain speed indicates it’s a problem with the tire. There is a chance that that you could have gotten a bad new tire. It’s rare but sometime the tire can developed a heavy spot in the molding process that even balancing can’t fix and only a new tire will fix that.

But if you don’t think it’s the tire then I would recommend taking it to your local 4x4 off road shop if there is one in your area. They deal frequently with tires and suspension and may be more familiar with this problem. Otherwise I would take it to your mechanic or local Toyota dealer.

Anyway, I came across this list that may help you trouble shoot this problem.
Toyota 4Runner steering wheel vibration causes and diagnosis