Bruce is right, the 6.75" axle is very weak. With gear ratios like you are talking about I would assume you are using a turbo 2.3 or a V8, and the stock 6.75" axle will not last long behind one of those, even with light throttle on a V8, it's just to much power.
Again, Bruce is right, the 8" from a V6 Pinto or Mustang II would be a better choice and you would easily be able to find parts for it. I run a Mustang II 8" axle with 3.00 gears behind my stock 2.3L and stock 4spd FOG transmission and it's great. Parts are readily available to!