I don't think there is a difference between M101A2, A3 I am fairly sure the A1 used a different bolt pattern. Having owned a military trailer A2 version I would suggest axle replacement to something that has the same bolt pattern as your rig so spares are universal. Other then direct replacement you are going to be limited in your choices and likely the choices you do have are going to be pricy. If you want to keep the original axle and you have a A3 I believe the bolt pattern is 8 on 6.5 bolt pattern. GM 8 bolt 1999 - 2011 3/4 - 1 Ton rims.
I'd go out and measure your bolt pattern to verify.