Where are the fill plugs? Surely it's not those small holes when you remove those plugs... How tf do you top off the fluid or check the level in these? Ugh. Oh yeah... It's a 2003 disco 2

 

How many miles on your truck? If 60 K or more you should be changing all the fluids.
Send me your tele number and I'll walk you thru the different steps of changing the fluids.
Here is a good list to use.
1 Oil filter, K&N 3001 or Mobil 1- 301
1 Factory air filter
6 Qts. of engine oil
2 feet of 3/16th vacuum tube
2 feet of 5/16th vacuum tube
12 Qts. of ATF
7 Qts. of 80/90W synthetic gear oil, 85W/140 for HD use
3 Pints of synthetic DOT 4 Brake fluid
2 Qts of synthetic Power steering fluid
2 Gallons of OAT, orange coolant, also called DexCool
1 Bottle of Lucas Power Steering additive
1 Bottle of Lucas Transmission additive
1 Can of Spray Carb. Spray
1 Can Of Brake cleaner
8 Spark Plugs, I use Bosch Platinum Plus 4's
1 Set of Magnacor 8mm plug wires
1 Serpentine belt
2 Bottles of SeaFoam gas additive
1 T/Stat
1 bottle of WaterWetter or Purple Ice
Hoses and belts, as necessary

60,000 Mile Service or Major Service for High Mileage Used Vehicle just purchased

Bleed the brakes
Flush and change the power steering fluid adding one bottle of additive
Service the tranny adding one bottle of additive
Change the coolant and t/stat
Do an engine flush before changing the oil
Do an induction cleaning
Clean the throttle body
Add fuel additive to full gas tank
Change both of the diffs fluid
Change the t/case oil
Replace the plugs and wires
Replace the air filter
Lube all the drive shaft grease fitting including the 2 slip shafts
Do full inspection of brakes, hoses, bushings etc.

Hopefully I didn't leave anything out.

 

You will need a 1/2 in drive rachet or breaker bar with a 3" extention.
Looking at the back of the transfer case you will see the parking brake drum, at the 10 o'clock position is the fill/check plug for the t-case.
Remove that BEFORE you drain it.
On the diff's there are 2 plugs, one screw in one on the bottom and a plastic plug toawrds the top.
That is your fill/check plug, the bottom one is the drain plug.
Both diffs and the t-case use gear lube.

 

I've gone through 2 T Cases on this truck and its a pain to find non destroyed refurbished unit

 

cause it has to be done underneath the vehicle. Plus its only 30k. Not all the time like Oil.

Just suck up the money and buy the necessary manuals or an online subscription to GTR.