1988 4.9 4x4 3.55 posi

How much and what kind in rear diff?
and do I need to change the front diff?
It so what kind and how much?

 

With an LS you can use 80w-90 or 75w90 synthetic with 4oz the traction modifier. The rear I think takes about 3 qt and the front takes 2 qt.

The front diff you will have to suck out as much as you can with a hand pump or drill and tap a drain hole at the bottom.

 

Does the cover not come off the front axle?

And should I just fill it all the way to the fill hole? Or just the specified amount?

 

very very doubtful you have a removable front cover on the front axle,
they are out there but i've only ever seen two and i'm not sure if they were factory, or the owner cut a hole and just put on a cover off a strait axle D44

and yes just fill it until it starts coming out the fill hole.

 

Do I need the ddditive in the front diff too?

 

No not unless its posi which likely it isn't.