1998 gmc whatis the deferance between the 2500 & the 3500 4x4 differentials , can i use a2500 front differential with a 3500 rear end a long the ratiosare th same ?

Owner's manual, troubleshooting? probably an electrical control problem in the differential.

Do you have the vehicle in 4 wheel drive? 4 wheel drive is only meant for slippery conditions. It will act exactly as you describe if on dry surfaces.

yes it has to be so the transfer case doesn't blow apart

i assume that you are talking about the seal at the front of the differential were the drive shaft bolts to the flange,if so this is called the front differential pinion seal,if your talking about the differential cover at the back of the differential(most rearward part of the rear end) this is called the differential cover gasket.

when was the last time you've checked the fluid level in the front differential?
Try this, quickly. these gears don't last long when the fluid level is low, and it's a real big job to replace the gears/differential housing.

I hope this helps, ask if you need anything.

Then the problem would be in the differential. something came loose or broke off the actuator on the inside

On the rear differential there will either be a metal plug or a rubber plug. The metal plug can be removed with a 3/8-inch drive ratchet. the rubber plug can be removed with a small screwdriver. Faceing the axle from the rear will see the plug to the right side on the cover plate. Remember to also check the front differential as well. Hope this will help you.

There should be a filler plug on top of the diff housing, it will have a square recess that will accept a 3/8" drive extension for removal.

Standard Differential, Rear W/ SGRX......GL-5 ALL TEMPS......80W-90
Standard Differential, Front......GL-5 ALL TEMPS......80W-90
CAPACITIES: Differential, Front..........1.5 liters Differential, With SGRX Rear..........1.9 liters Saginaw rear axle has a removable rear cover without fillplug. Differential, Without SGRX Rear..........1.8 liters

check all lines some have vacum or small elc. motor on front gears