Just so happens we (the Whistlepost & Milemarker crew) were out at Big Horn doing some milepost and signage work during "C" session along with ...trying to discover how to drive to Whiplash.
If you know how to get to Big Horn you can get to either the upper crossing near MP297 or down at the bottom crossing near MP296. Unfortunately the "roads" are somewhat difficult to find and or follow. They are, however, both USFS roads and 4WD is not needed. That being said Whiplash is not the place to try and get to with the rental Ford Focus. You'll need at least some ground clearance and most 2WD pickups or small SUV's should have no problem. Yes, you can get up to Lava from Antonito via Ortiz (up past the cemetary) but unless you like fording melon-sized (and larger) boulders the route is not for the faint of heart. Your kidneys will be down around your ankles and you'd better wear a hard hat when you drive. We've been out there (Lava siding and the tank) this past "C" session as well and it is not a pretty road.
A great tool is using Google Maps in the satellite mode. Get yourself to Big Horn and you can easily see the roads I mentioned. The only obstacle to ford when going to the lower loop from Big Horn is the track crossing. You may have to get out and place some old tie chunks to help ease the way over the tracks.