Looks like 4-Runners have always come with at least 16" wheels. You will have to do something to make more clearance as 16" trailer tires come in 80 & 85 profiles and are quite tall. Unless this 4-Runner has some aftermarket 15" wheels, you are probably going to have to do a lot of work and end up with results you aren't happy with.
Two 14" 5 lug S5 wheels with ST205/75R14 Kenda Loadstar radials $349
Two 14" 5 lug S5 wheels with ST215/75R14 Kenda Loadstar radials $389
Two 15" 5 lug S5 wheels with ST205/75R15 Kenda Loadstar radials $409
Mounted, balanced and delivered.
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CARBURETOR REBUILDING - $199
Parts, labor and return shipping included
2 LED 6" BRAKE LIGHTS DELIVERED $32