The Grotto is located off the Lost Creek forest service road, on Terepoki Creek.
The trail up to the Grotto pool requires crossing Terepoki Creek, so the difficulty varies tremendously depending on the water level. After a dry summer a stock vehicle on 30" tires should have no problems reaching the pool, as the water will be low and you can see the rocks.
When the water is just high enough to hide the rocks a stock rig can still make it, but you need a leader who knows the route very well so you don't leave the smoothest route and get hung up on taller rocks. Under these conditions dragging your axles occasionally is almost unavoidable and skid plates on vital components like your gas tank are a good idea.
When the water is higher in later Spring or in the Fall the trail should be only attempted by drivers who know their rigs well enough to know whether their vehicle capability matches the water depth.
Note that the trail to the Grotto pool (sometimes called Grotto Base Camp or Brah Base) and "Grotto Trail" are not the same thing and are often confused. Grotto Trail is a hard-core trail that loops from the East side of Grotto pool up into the trees and back down to the creek. It is for modified rigs with 35"+ tires, dual lockers, and full skid plates.