If it is a swapped in axle or if it is lifted and you adjusted the pinion angle by rewelding the perchs or using shims, you will generally get a little less in. In an extreme case on a jeep I did gears in, The owner over compensated for stupid high lift on a short wheel base by having the pinion rotated up so much that not enough fluid was in the diff and it starved the oil slinger and smoked the pinion bearings. If this is the case, I suggest parking on a slope with the front facing down or jacking the rear off the ground enough to get more fluid into the diff. It's kinda hard to do, but you need to visualize a straight line from the bottom of the fill hole to about middle of the pinion oil galley. if you perceive the imaginary straight line to point up facing foward (the fill hole is lower than the slinger) you might want to force some extra fluid in using the previously mentioned methods.