Like Bill said, $300 won't cover the cost to rebuild, unless you can get the labor done for free. Gears alone are $150-200, then you need an install kit, which includes bearings, etc and that's $125-$175. Then there's labor. I've seen anywhere from $150-$250 charged for it. So, for brand new gears installed, the minimum would likely closer to well over $400. But more likely in the $500-600 range per axle.

One advantage of the factory 4.88 gears is that you'll get a V-6 rear diff, which is stronger. The disadvantage is that they are harder to regear to something like 5.29s if you want to use the same carrier. (I think)

The other disadvantage is you never know what condition they are in. The biggest problem with high mileage Toyota diffs is bad bearings due to lack of changing the diff fluid. Seems everyone remembers to change their engine oil, but most folks had no idea that diff fluid needed changing too.

As for being rare, the 4.88s are sort of rare. They were extremely common overseas, but here in the states they only came on '92-'95 trucks and 4Runners that had the combination of a V-6, automatic trans and 31" tires. There were plenty of 4Runners that had this combination, which is probably where most of them come from, but not as many trucks. As time goes on, the 4.88s will only be harder to find.

Not sure about the ABS sensor and how it would affect things.