The other day I went out to get in my truck after putting a new rear differential in it, and it would drive forward but reverse worked perfect, you can feather the throttle in drive and it will barely move but if you shift all the way down into 1st it will put the truck in reverse! Any help will be much help!

 

so no forward at all, just reverse.
maybe check linkage adjustment

 

What year and model is your truck?
Did you replace the rear axle with the exact same model?