why does my speedometer jump when i do 50mph after replacing the whole rearend

on 1990 Ford F-150

changed out the whole rear end and now the speedomoter jumps when i get to 50mph.It jumps to 100 then back down etc.

2 answers
check the differential to see if you have a speed sensor near the left rear wheel which might not be working correctly with the computer . The one you put in might be bad.