Your 4Runner probably had 15" wheels as the stock, base offering...check your door jam sticker. My Mom's '98 has the 16" factory wheels with 265/75R16 tires and the spare is also 16" but the sticker on the door jamb says 235/75R15 under tire size. I wonder if they all had 15's on them for the boat trip and then were swapped out for the American market.
I've only run 15" steels on my '85, in widths from 6-8" (currently 6") without any complaints, except for one set (see below). 31x10.50's are a nice, all-around size with plenty of sidewall for trail exploring and good availabilty...all tire shops (and many garage sales) have them.
Herdnerd...I did have a similar experience with a set of wheels that I bought from someone's back yard. They always zeroed perfectly on the balancer but had substantial wobble over about 45 mph. I think one of the wheels was slightly bent. If you can have someone drive next to you and watch, sometimes the wobble is visible and you can find out which wheel(s) is/are giving you trouble and swap them out. That can happen with any size wheel but especially steel. An aluminum would probably crack with the amount of force it takes to bend a steel wheel out of center. I swapped on a different set of (used) wheels and haven't had that problem since.
Eric