03 Ranger with problems

My dad owns a 2003 Ford Ranger XLT 4.0L (4WD), and he's having some problems.

He's had two drive shafts replaced, and one was lubricated at the slip joint.

He feels a slight hesitation when starting to accelerate on a hill, and wants to know what the problem is.

anyone know what could be up with it?

thanks in advance.

wich drive shaft's were replaced? rear, front, half shafts? why were they replaced? slight hesitation as in motor?