I have a 98 f150 4.6 with a new rear in it. It was just rebuilt so I dont think it is the problem. At about 40-50 mph the driveshaft whines, but under 40 and over 50 you cant hear a thing. Could it possibly be the u-joints or is there something I am missing. Thanks in advance for any input.

 

Most likely it's ring & pinnion gear noise. Since you said the rear end was rebuilt, you might want to have the rebuilder re-check the pinnion gear depth & ring gear backlash,also have them check for ring gear run out & gear tooth contact for a aceptable pattern. Any of these can cause the noise you described.