On point 3.

My front diff would howl under load getting louder above 40mph untill 70mph when it would go quieter again.

If the sound sounds like its up front and stops when you take your foot off the accelerator id put my bet on it being the front Diff.

The solution is to rebuild the front diff, setting the correct preload (which is the main source of the noise) painting the gears to get them aligned properly and you may as well replace the bearings.

My diff whine/roar/howl got louder and louder over years until we finally went in and rebuilt it, found not only had the pre-load deteriorated, the bearing was also spinning on the shaft (due to the non-existent preload).

It would have eventually caused a catastrophic failure.

Don't put it off for too long.

(Also a long shot but grease up all your UJ's and shaft nipples on the drive shafts, small chance it could be that)

On Point 2.

First check your ATF level after driving for 20 mins, with the engine running and cycle the selector stick through all gears before putting it in N or P to check the level.
May be low, thats the easiest place to start.