It will cost you more to regear with 3.73s now and do the 4.56 later. Since you want the 4.56s, do it now. You can sell the 3.73 gears and diff later to recover some of the costs.
I bought my last couple of sets of gears from www.completeoffroad.com , they were selling genuine Dana/Spicer or Yukon for the same price. Most of the gears available are made in a couple of plants, and boxed under different names anyway.
The carriers you have are good for 4.56s, so if it were me, I'd get the 4.56s, regears both ends, and be done with it. A little more work and expense, but once done, I'd be happier.
Bob
tufcj
