Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
If the engine doesn't runaway and rev on grades the clutch should be good. But if the engine speeds up and the truck doesn't then you need to look at the clutch. If the pedal is real high with no free play the clutch adjustment might be too tight. This can make the clutch slip when the clutch is still good. If this is happening, loosen the adjustment up until you have an inch or so of free play. Just keep an eye out for the engine speed revving way up while the truck isn't accelerating. That is a sign the clutch linkage is too tight, or the clutch is about to lay down on you.
As far as the sound and the rear end. That test on the oil can tell you if a bearing or part buried deep inside is failing, something that you cant even see with an inspection camera. I cant overstress how much this one test can tell you. I have gotten to where I count on them a lot, and they have never let me down. It saves a lot of tear down time, and it can spot some problems that are hard to find in a tear down.
I would evaluate the clutch and its adjustment, go over the rear end and the transmission, back wheels up in the air, on jack stands, engine running and in gear. See if you can find the sound with the stethoscope. The consider $35 for the spectral analysis on the oil. You really get your moneys worth on that test. Remember Blackstone Labs
on the internet. They send you the kit to take the oil sample free, the test cost $25. If you have any more questions at all, or if you want to talk on the phone, we can do that under additional services. I will be available after 3PM today. Right now I have been up all night working, and am going to catch some sleep time. I will be back on this afternoon. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer. I do not get paid for my work unless you rate at 3 to 5 Smiley Faces or Stars, a bonus is always greatly appreciated. If the answer is not clear, let me know what additional help you need, and I will be happy to assist you further.