I only know of one gear company that has a life-time warranty, Genuine (crap) Gears. The catch is a large part of the cost of replacing gears is the labor and that's not part of the warranty. So it becomes a question of how many times do you want to pay the mechanic to replace your free crappy gears. It's better to just buy good quality gears to begin with and only do it once.

As far as 5.29 gears not lasting as long as 4.88 it is probably true to some extent, but I wouldn't run the wrong gears just because they might last longer than the right gears. The 5.71 do have a bad rep for not lasting very long but I ran 5.71 Genuine (crap) gears for 40K miles before they started making noise.

I am running US Gears now.

 

4.88s only came from the factory on *some* automatic transmission 4runners.