i have a 98 gmc sierra 4x4 and when i drive my truck it makes a humming noise when if crusing and when im going slow like 15-20mph and just coast it makes a grinding noise,

ive just had new gears and all new bearings n seals put in last august
when the gears came apart, just wondering what it could be, truck is lifted 6 inches and has 3inch body lift

 

I would check your universals first, but it sounds like the gears/bearings may have failed.

 

You have to have some experience to properly rebuild a rear end. Did you check the differential fluid by any chance? U-joints usually make a very distinct sound,but it shouldn't be confused with a "grinding" noise. Check the rear differential fluid first and see if it is low,or if it has a funny smell to it.