please give me some tips on changing the fluid. also how difficult is the diff. fluid to change?

Here's how I did mine:

1. Get trans kit from O'Rielly (Screen and gasket)
2. Spread LARGE piece of cardboard under ZJ
3. Place oil drain pan under tranny
4. Remove alternating bolts around trans pan (may start dripping)
5. Loosen remaining bolts (should start dripping pretty good here)
6. Remove all bolts but two on opposite sides to support pan
7. Allow pan to tip a bit by loosening one bolt more than the other
8. Hold pan up with one hand while removing last two bolts
9. Lower pan and drain into oil catch pan
10. Remove two torx screws and pull screen from tranny
11 Clean pan (all of it) and gasket mating surface of tranny
12 Install new screen and reinstall pan (I've found it handy to lay the new gasket on the pan and start the bolts into the gasket. The bolts hold the gasket in place and the gasket holds the bolts in place)
13. Tighten pan bolts in a criss-cross patern to spec (it doesn't have to be real tight, about what you can do with one hand and a 3/8" ratchet)
14. Drain fluid from oil catch pan into milk jugs
15. Add new ATF+3 to tranny to match the amout measured in the milk jugs.
16. On the side of the transmission to the rear there is a steel tube held in place with a simple plastic catch. Push this tube in, squeeze the plastic connector and pull it all out of the tranny (this is the return line from the cooler)
17. Slip a hose over the end of the tube and route to a milk jug
18. Start the Jeep and move the gearshift through the gears until the jug is nearly full (It won't pump much at all in "Park", let it flow in the gears and neutral)
19. Add 4 quarts of new ATF+3
20. Remove the hose and push the steel tube back into its hole in the tranny
21. Clean up the mess, drive the Jeep a bit.
22. Check for leaks and top off trans as needed
23. Congratulate yourself, your done!

I used 9 quarts of ATF+3 for this and figured I got 90% of the old fluid out. You could repeat steps 18 and 19 for a more thourough flush, but it's probably not necessary. Check the pan bolts after a week and retighten as needed.

The best way to change the fluid in the differential is to pull the cover to drain the old fluid. Replace the cover with a generous helping of RTV and refill. I suppose you could suck the fluid out with a hose and a turkey baster, but it wouldn't be as much fun.