How many miles can i drive my 2002 gmc 2500 hd before i should change my spark plugs thanks

SOURCE: GMC Safari Van - Change spark plugs

i went under for driver side then i took off passenger side wheel you also have to remove two bolts that are holding dipstick tube in place it is easy to get them from the wheel well on passenger

SOURCE: How do I change the spark plugs in a GMC Envoy

Location of spark plugsThey are located along the top of the motor under coil packs that fire the plugs. You have to remove the resonator box from the top of the motor to gain access.
Each coil pack is secured with two long skinny bolts and must be removed carefully. Then simply lift it off and remove the plug directly underneath it.
Use only AC Delco iridium plugs. 100k mile intervals.

SOURCE: 2005 GMC 4x4 HD 2500 6.0 eng. rear axel seal

SEAL COULD BE LEAKING CAUSE YOU DIDNT LUBRICATE THE SEAL LIPS WITH PETROLEUM JELLY TO HELP THE THE SEAL LIP SEAL OR YOU DAMAGE THE SEAL WHILE DRIVING IT IN.AXLE FLANGE NUTS TORQUE IS 58 FT LBS THE AXLE HUB NUT TORQUE IS 177 FT LBS.