on a 87 bronco11 4x4 if I change the rear diff ,do I need to change the front diff also?
I was told that it may not match.

As long as the gear ratios in them are the same it doesn't seem to me that it would make a difference.

Well this is the thing, the number on both rear diff is different(the # on the tag that is)

Both front axles need to be the same ratio. If you have 3.73's in the front and rear and you change the rear to 4.10's, then the front also neeeds to be changed to 4.10's. What are the tag IDs?

Present axle:
Replacement axle: