Front & Rear Differential Service - 95 Rodeo

I have a '95 Rodeo 3.2L 4WD automatic with 103K miles on it. I bought it used with 44K miles 7 years ago. While I change the engine oil every 3K miles and the tranny fluid every 25K miles, I can't remember the last time I changed the front & rear differential and transfer case fluids. Maybe 40K or 50K miles ago? Anyway, I think I need to do it now because a slight "cupping" noise has developed in the rear end when I accelerate from a stop. I have several questions for the forum:

1. About 5K miles ago, I had the front half shafts replaced. Would the mechanic have had to replace the front differential fluid then? (Meaning I can pass on it now.)

2. I just learned today my Rodeo has the G80 limited slip differential (maybe all 4WDs do?) The owner's manual says some kind of additive needs to be added when the fluid is changed. What brand do you guys recommend?

3. I can't do this work myself. How much should I pay to have all three serviced (front, rear, and transfer)?

4. Any other advice related to this work?

1. No, they wouldnt have needed to drain and refill the diffs to replace the half shafts. Unless they also replaced the "cups" that go into the front axle housing. Then they would have had to drop the front axle and open it up. There really isnt any need to do that unless the "cups" are cracked or broken.

2. I used Valvoline SynPower 75w90 synthetic gear lube in my diffs. It already has LSD addative in it so no need to add anything else.

3. No idea on cost. I've always done it myself on all my vehicles.

4. The xfer case uses 5w30 motor oil instead of gear lube (like the 5 speed manual tranny). I used Amsoil MTF Synchromesh 5w30 in mine (as well as in my trans) since I dont like using engine oil with gears.