Yes, the "Anti-Skid Recovery" is designed to prevent wheel spin, which represents loss of traction. This is a very smart design for typical on-road slipping scenarios, when both wheels have enough surface contact. But if the terrain is challenging enough, such as when one wheel is lifted (such as in the typical off-road, crawling over rocks scenerio) or sometimes in snow/mud (where a little wheel-spin can be helpful to dig till there is traction), the ASR system will simply prevent the van from moving no matter how much you rev the engine, because it's designed to brake any slipping. Even backing up my driveway with a little wet leaves and mud I've found that turning off ASR allows me to crawl up it with better control (vs the ASR stopping the van and losing momentum as soon as it senses slip). Momentum can be a key ingredient to getting through low-traction scenarios, and ASR braking sometimes prevents this.
As Shibby explained, a locking diff will mechanically force both wheels to spin at the same time, which has much better results in off-road or more challenging scenarios because it equalizes torque across drive wheels, allowing them to push the vehicle from either wheel where traction is found.
For my purposes I would consider a rear diff lock upgrade over a full 4x4 conversion. Has anybody in the US had any success with this?