Your D100 came with a 8 1/4 axle
Look at the rear lower end of the engine
sure grip
Replacing a rear differential seal is easy. All you have to do is unbolt the shaft and take out the old seal.
IF THIS A SMALL BLOCK LA V8 ENGINE, THE OPS IS PROBABLY LOCATED AT THE REAR OF THE BLOCK NEXT TO THE DISTRIBUTOR ASSM.
You have to remove the tranmission and the flywheel.
You remove the trans and flywheel. The seal then drives in from the rear.
Most are located inside the rear section of the alternator Dodge usually mounted the regulator in the center of the firewall, until the trucks were fuel injected then the computer took over the job of voltage regulator.
If you have to ask, the answer won't help you.
rear main seal leaking
5-6 hours
V8 , no. Diesel, yes.