On your truck, there are a couple of possible causes for gear oil showing at the vent line either in the front or the rear differential.
The most common cause is over filling. Do not be afraid to have the level at the inspection/fill plug be down a bit. The next step is to be sure that the correct gear oil is used so that there is no possible foaming that would create pressure to push the fluid out the vent.
Another suggestion on the front differential is to replace the vent line adapting to a slightly larger hose at the upper end of the vent line, and relocate the vent slightly higher so that the differential must move the vented fluid farther up the vent line before it can escape.
But, number one is do not fill it right to the top and use the correct recommended fluid.
Below is a service bulletin on rear axle lubricant recommendations.
File in Section: 04 - Driveline Axle
Bulletin No.: 99-04-20-001
Date: June, 1999
Subject:Rear Axle Lubricant Compatibility and Usage
Models:1999 Cadillac Escalade1988-99 Chevrolet and GMC C/K, G Van and P1-3 Series Models Built after the VIN Breakpoints (C/K) listed.
This bulletin is being revised to add the 1999 Model Year, VIN Breakpoint information, and additional text Please discard Corporate Bulletin Number 86-42-04 (Section 4 - Driveline/Axle).
This bulletin is being issued to inform dealers about the compatibility and usage of the new Fuel Efficient SAE 75W-90 Synthetic Axle Lubricant, GM P/N 12378261 (In Canada use P/N 10953455), with current axles, and with the non-synthetic axle lubricant, GM P/N(NNN) NNN-NNNN(In Canada use P/N 10950849), for the rear axles on the above light-duty trucks.
Testing has shown that the new synthetic lubricant will chemically attack the Room Temperature Vulcanizing (RTV) sealant. Use only the non-synthetic lubricant, P/N(NNN) NNN-NNNN in axles using RTV sealant (See 9.5 and 10.5 information below).
American Axle and Manufacturing (A.A.M.) has approved the use of the new Fuel Efficient SAE 75W-90 synthetic axle lubricant for rear axles with the following restrictions:
On vehicles built after mid-February 1998 and equipped with 9.5 inch rear axles, synthetic lubricant, P/N 12378261, can be used because the RTV was replaced with a gasket. Other mid-February changes to the 9.5 inch rear axles include a new cover pan and cover pan fasteners common to other axles.
As a product improvement, this new lubricant has been released into 1999 current production C/K vehicles built after the above VIN breakpoints (Except HD3SOO models which use 75W-140 synthetic lubricant, P/N 12346140).
It is recommended that synthetic lubricant be used when changing rear axle lubricant for the following reasons:
Parts are currently available from GMSPO.
I hope this information is helpful.
Have a Great Day