Designtech does make 50W halogen replacements for most cars that are available at autozone, pepboys, etc.. but I've not seen the bulb type for the 4runner.
Drawback to using the stock backup lamps is that the backup light is somewhat small, and being integrated in the taillamp housing, it's mounted high... the aux backup lamps put out a much brighter/wider beam so you can actually SEE when backing.