Best recommendations for ATF and gear lube? Also, should all of the u-joints be repalced with greasable one's? It appears that only my front one is.

2000 Disco II 115,00 miles

 

ATF I use Valvoline Maxlife full synth in my DI. Gear lube, I haven't changed mine yet, but I know Amsoil makes some good 75w-90 full synth.
And as long as the U joints in the front double cardon are greasable you should be fine, make sure to grease them at every oil change.

 

Yes, all u-joints and replace the centering ball kit as well.
Gear oil should be synthetic for the diffs and t/case. Try using Vavolines high mileage Dextron .