Depends on what you're doing with them.
I'd recommend getting a spare differential for the gears. I have 3 that I can switch back and forth, depending on my mood; 2.73, 3.27, and 4:30.
IIRC you want the 28 spline IRC center section. If you have one of those, you can swap it in yourself in about an hour, no messing with gears. It will set you back a few hundred dollars, but it's worth it in the long run.
Link for a 4.10 unit.
Link to an entire spare Mark 8 rear end.
Busy this weekend, Eil? Let's tear open my 2.73 and find out why it sounds like it full of gravel.
Hey wait, does that pricign offer apply to me? I have a 3.55 track lock chunk sitting here, I'd like to gut the 3.27 pumpkin and put it in there.