I need to know how to replace lower ball joint in my custyomer's gmc vandura 1996 model 6.5 d 3500 series.

Hi, the ball joint is connected to the steering knuckle with a tapered connection, called a steering taper, which is also used in other steering connections.
To remove a ball joint requires the use of a ball joint remover.
Another way of "breaking" the taper is to use a large hammer.
First, remove the cotter pin from the ball joint nut and loosen the nut several turns.
Position the vehicle so that the coil spring is pushing on the ball joint. This could require lifting the vehicle or allowing its weight to rest on the wheels.
Use a hammer to pound sharply on the steering knuckle on the outside of the taper. This will deform the taper and spring pressure will separate the ball joint from the steering knuckle.

Removing the Ball Joint from the Control Arm
A ball joint can be retained by one of several methods.
Some ball joints are held in with rivets from the factory. The rivets are drilled and punched to remove them. Bolts and nuts are used to hold the replacement joint in place.
The most common method is press fit. Some manufacturers require you to replace the entire control arm assembly if a ball joint is to be replaced. In these cases, the ball joint and control arm are made as a single assembly and individual parts are not available.
If the control arm has been removed from the vehicle, a pressed-fit ball joint can be removed using a standard hydraulic press. When the control arm is still on the car, a special press set allows the removal and replacement of these ball joints. Some pressed-fit ball joints have a spot weld that holds them in place. This must be carefully removed. When a replacement ball joint is installed, a snap ring often takes the place of the weld.

NOTE
Be sure the grease release hole in the new ball joint's rubber boot is aimed away from the brakes.