wow
4.55's...
The closed knuckle dana 44 is near impossible to add disc brakes too, open knuckle is easy. There might be a kit out there but i'm not familiar with one. You have a more limited turning radius as well. Nothng wrong with the axle unless it's leking gear fluid near the inside of the wheels. The best setup that guys swap to is the 14 bolt rear and dana 60 front from CUCV's (k-30 military trucks). You can find those setups for about $1500. Then you can easily do 4 wheel disc if you wanted.
diy4x.com and offroaddesign.com have a lot of great parts...
The rockwell is a good transfer case, but parts are more expensive since they stopped using it in 69, the 205 is more common. I'd run it "as is" for now and make sure everything has fresh gear oil in it (including the TC). The rockwell is also noisy compared to the 205 so you might feel like you are driving a tank with the in cab noise...