I have 4.10 in my 89 8-lug 2500 with the SM465 it pulls really well.
Also have 4.10 in my 96 K3500 Dually with the 454/4L80E combo I get at best 10.5 MPG if I keep my foot out of it.
Any corrosion is a bad thing. Pulls the voltage down considerably and creates heat which further pulls it down. ANY CORROSION in the distributor is not good either, especially on the pickup coil which is just below the distributors rotor. Its 2 wires that can be Ohm checked relatively easy...
Check the back of your intake manifold really good where it meets the block and heads. If you can use a flashlight and a mirror it should be easy to see. They are notorious for leaks in this area. It will leak and come right down the bellhousing of the transmission simulating a rear main seal leak.
Agree with df2x4. Those plugs where the wires pass thru the trans case are known for leaking.
GM sells the replacement harness and o-ring setup to stop that leak. They can leak driving or sitting. Unfortunately the pan must be removed again to replace the harness.
I run the yellow and blue 4600's on my 96 K3500 Dually. Best ride I've had in it. I did try the KYB Maxx but they blew out within 2 months, they don't like heavy towing! No issues with the Bilsteins in the year since I put them on.
Check your cap and rotor real good. I've seen one cylinder get affected from a bad cap that was new. Check it for carbon arcing inside. Also when dark pop the hood, have someone load the motor and spray water from a spray bottle on your wires, if you see any arcing your plug wires are leaking...
A lot of issues arise with starters because NO ONE torques the starter bolts. When replacing I advise to torque them, especially on high torque aftermarket starters.
Front differential is used as a crossmember of sorts. It helps rigidity in the front frame area, I would not remove it. Remove the CV axles and front driveline, but not the diff.