The front axle is engaged by vacuum. Most common is a cracked or leaking hose but can also be holes rusted in the vacuum canister. This canister is mounted on the front axle, passenger side. Sometimes they just get gunked up and need cleaned and lubricated to get them working again. When it is working properly the vacuum pulls a fork inside the axle. That fork slides a gear that connects the wheel to the differential axle. So without vacuum the 4x4 cant engage. In most cases it comes down to issues near this canister. Check it and see.
The front differential is an "open" differential. No limited slip components there.
The front differential uses 80w90.
Check on the internet fat cow
You unscrew the plug on the front cover, and ad 75w90 to the bottom of the hole.
Dana 216 FBI (Dana 44)
if 4x2 only 1 differential which is the pumpkin between your rear axle, if 4x4, there will be a transfer case at the front of the vehicle coming off of the transmission going to front drive shaft and front differential.
Front axle is a 9.25 American Axle Rear axle is a 10.5 American Axle
75w90 synthetic.
There is a diPstick at the front on the transmission differential
The actuator of the Dodge Ram 4x4 is located on the top of the front differential. Depending on the year, the actuator is either vacuum or manually operated.
You can find the schematics for a front differential on a Jeep Grand Cherokee 1994 at various auto part stores. You can also check in the owner's manual.
Check the vacuum line to the front differential.