A used hub to hub axle is the way to go. If your truck is rear wheel drive, it is a fairly easy job.
The rear on a 4 wheel drive is easy too but the new rear end must have exactly the same gear ratio. To check , jack up your rear wheel in neutral, put a chalk mark on the drive shaft and rotate the off the ground rear wheel twice, The number of turns the drive shaft makes is your axle ratio.
The front differential on a 4 wheel drive is a lot more work and it may be cost effective to rebuild the differential but be careful who does it. Not all mechanics can do it and have it turn out well.
I does sort of sound like differential noise, likely due to pinion or ring gear wear from being driven with low axel oil. No harm done to try the additive thing now the seal is replaced and fresh axel oil is in there. You used the correct axel oil, right? It meets the manufactuer’s specs for the vehicle? Even if you have wear, it may not be necessary to replace the whole thing. An adjustment might fix it. Usually done by shimming the pinion/ring gear for proper clearance.
Usually those limited slip friction modifiers show their benefits on slow speed turns, minimizing weird noises and clunks when slowing to a stop, and leaving from a stop, not buzzing noises when letting off the accelerator at higher speeds. But since trying it is low cost and easy to do, it’s worth a shot.
One other idea, the low axel oil might have damaged a wheel bearing rather than the differential. If I had this problem on my 4x4 truck I’d be inclined to disconnect the drive shaft (easier to do than it sounds), jack the rear end up, and turn the wheels by hand in a quiet environment, see if I could hear something suspicious.