rear gear whine

[tarafied1]

I bought this old 72 as a driver for my boys with plans to fix it up if it survived them. It's been doing pretty well but I haven't really used it as a truck until last weekend. I bought two 69 Mustangs out in Eastern KY and used the 72 and my dad had his F250. We drove for hours just to get to them but shortly after leaving the house with the trailer hitched up, the rear started whining and was really hot. I checked the lube and couldn't feel any so I put a pint in and that filled it. I stopped frequently to check the rear and see if it was till getting hot. It seamed better but still noisy. It made it all the way out to Eastern KY and back loaded with a 69 Mach 1 and lots os parts. I let my dad tow the heavier load with his 250. The rear doesn't make much noise around town or with no load but as soon as I work it it is really loud. I think it might be the gears because if it were a bearing I would think it would be noisy all the time.