XREAS shocks have hydraulic lines coming out of the top section of then, regular shocks don't. You can see this really obviously looking at the rear shocks.
If you look at your frame rails, you will also see metal pipes running forward and back along the outside of the rails. Those are the hydraulic lines, although having them the doesn't mean they are hooked up to the shocks.
Only the OEM XREAS shocks will keep the XREAS system working BUT there is no harm installing Bilstein shocks, it will be exactly like if you had installed the Bilsteins on a non-XREAS 4runner.
I have actually never driven a non-XREAS 4 runner, but some people say the good Bilsteins are better than XREAS, and others say it's almost but not quite as good. Everyone says installing normal shocks is definitely the cheaper/simpler/more mod-friendly way to go.
I just bought a 4runner with a bad XREAS system, the dealer insisted on fixing it before I could buy the truck, instead of just giving me a discount. This was a Volvo dealer, and the local Toyota dealer wanted $1800 in parts and 12 hours of labor to install 4 new XREAS shocks. The dealer ended up having an independent shop install them for $2000k.
It rides nicely, but I would rather have had them install the Bilsteins and save me some money. I bought the truck anyway because it was still a good deal.