GM 14-bolt FF Rear End with 149k miles

Mine is the same, except 1996, 4.1's and over 165K miles of extremely
hard driving/towing/offroading

GM's G80 is

NOT

a limited slip, but a LOCKER with clutches

It does NOT need, nor does GM recommend any LS modifier

LS Modifier reduces the clutch pack from sticking...exactly what the G80 wants to
do when the centrifugal switch turns on the locer

Recommend you flush and refill with synthetic diff fluid WITHOUT any friction
modifier for LS diff's

I've used Mobil synthetic since new.

A trick, as this era does NOT have a drain plug.

Remove the diff cover and flush.

Scrap off any old gasket on the diff housing and cover.

Degrease it well

Then glue (Permatex) the new gasket on the diff cover side.

Lightly coat the diff housing flange with diff fluid. Ditto the gasket
side that will mate

Then reassemble and tighten the diff bolts 'lightly'

DO NOT FILL

Leave it this way for a couple hours...best over night

Then tighten the bolts to sped and refill

Next flush: just loosen all of the diff cover bolts, but leave them
on

Place a catch pan underneath the diff pumpkin

Then whack the diff cover. Normally dead center, but on GMT400's the
gas tank is very close and in the way

The diff cover will just pop off and a gush of fluid

Then take the bolts off.

Wipe the diff cover and gasket

Let it drain and then repeat the wipe of fresh fluid on the mating
surfaces.

No new gasket, as the old one will be in one piece.

Tighten and then refill...done

Flush mine every 20K miles or two years...which ever comes first