You could go with a grade 12.9 5/8" SHCS. (socket head cap screw) such as is used in machine shop assemblies.
My gut feeling is, you'll spin the tires bald on a 4Runner before you shear a 5/8" grade 12.9 SHCS.
A 4 runner ain’t snapping no 5/8 grade 8 bolt.
We move campers with forklifts. We have a 1", thick steel plated bolted to the forks with 4 large grade 8 bolts. They have sheared in the past when a side load was forcefully applied. I promise you a 4 runner can break them in the correct situation
