I've got a 94 F-150 with just 100k miles. 6cyl E4OD. The rear axle is howling pretty good at lower speeds. My mechanic says about $875 for a rebuild. I had given some thought to doing a boneyard axle swap to save some money. My axle ratio is currently 3.06. If I swap for a 3.31(worth it?) or 3.73, what will I need to do to get my speedo to read correctly?

 

Bone yard is a good choice.
To find out how far off the speedometer is, I'd always pace with a buddy at 60 and note how far under/over I was. Then go from there.

If you go with the 3.31, I wouldn't think the speedometer would be off by more than 2 or 3 mph. I used to have a chart in my toolbox for this.
By going to a 3.31 or 3.73, Your speedometer in this case would be showing faster than normal.
The speedometer can be corrected by changing the plastic transmission gear where it connects to the cable in the side of the trans.
For example, If you have a 16 or 18 tooth gear, To correct a fast speedometer situation, Go to a 14 or 16 tooth gear to make the cable turn slower.


If it is way off you can get calibration units.

 

https://www.ford-trucks.com/article/...ND_BRONCO.html

Or, just do it in the dash, with no "recalibration"