I recently picked up a salvaged rear axle which is identical to the one I'm replacing except the ring gear is an 8.8 instead of the original 7.5. Question is; what adverse affects, if any, am I up against? Will it simply be a difference in the speed indicated or some other issues also?

 

If they are the same gear ratio then no problems, if not then just the speedometer will be off. Other than that you shouldn't have any problems.

 

If the gear ratios are different, you still can do the axle swap. You can get the correct speedometer drive gear from a Ford dealer or at shop that services speedometers.