I just picked up another 99 f350 cclb 4x4 7.3 zf6 just like my red truck. It has a couple issues but price was right and its super clean. My question is would it be speed sensor in rear end causing the speedometer to not register untill around 40mph?

 

That is a possibility. Try cleaning the VSS and see if that helps.

 

Ill give that a try. I can always swap from my red one to try also