My X-REAS failed on my 03 Limited at 138,000 km (or 85,750 miles). There was a leak in the system - in the line behind the Driver’s Side Centre Control Absorber. Leak was likely caused by corrosion of the line due to road salt used up here during our Canadian winters.

Fluid that had dripped onto the driveway was the first indicator of some sort of problem. As more fluid leaked out of the system, the suspension became worse.

I removed the entire X-REAS system (too costly to replace), and installed the standard SR5 suspension (to keep it relatively stock).

In hindsight, I might have sourced a quality aftermarket shock that was less expensive than the OEM Toyota shocks - but I did not have the luxury of having time to "shock shop" due to the nature of the X-REAS system failure.

These links/threads might help:

Toyota 4-Runner X-REAS Shocks - Shockwarehouse.com

Best Replacement Shocks when removing the XREAS? - Toyota 4Runner Forum - Largest 4Runner Forum

Replacing XREAS with regular shocks - Toyota 4Runner Forum - Largest 4Runner Forum

5th Gen Limited X-REAS Leak - Options? - Toyota 4Runner Forum - Largest 4Runner Forum

At least now I'll be better prepared if/when my X-REAS goes on my 2011 Limited.