The price definitely seems too good to be true. Unfortunately, I thought the TRD exhaust only fit the XREAS-equipped 05/06 4Runners with the spare taken off.
With the spare intact, I think your best options are Magnaflow or Borla (haven't heard any reviews about Gibson). Borla recently released a dual catback exhaust for XREAS-equipped 05/06 4Runners. It's very similar to the TRD system, but works around the spare tire so you can keep it intact. This kit is available from Borla as part#140146 and retails for just over $1000. It is also available through Southeast region Toyota dealers as a port or dealer installed option... part # 00016-89008, which retails for $779.
I just ordered a set from Courtesy Toyota in Tampa, Florida. Spoke to Tad Bowling in the parts department and he got me the set for wholesale... $679, plus shipping to California (where I can't find the same system except through Borla). Even $679 seems expensive for exhaust, but it's on par with what the TRD system usually costs and you're still getting a nice stainless system with engraved tips, like the TRD. Check out X6RUNNER's system on his 06 Sport Edition (in the "test-loading pics" posting). He seems pretty happy with it.
If anyone else is interested in this Borla system, PM me and I can get you Tad's contact info at Courtesy Toyota. Nice guy and very professional to deal with. I'll post pics and my impression when it gets here and I bolt it up.