Hope you guys can help.. after changing my rear axel my spedometer will not work, and the OD light keeps flashing on and off. is this because of the speed senser? also at tmes it revs high before shifting into second

 

What year is your truck? I assume it's '92 or up. The speedometer and ABS sensor are in the rear end, and if it's not working correctly then the transmission will not shift correctly. The speedometer and ABS use the same sensor.