Rear End Hummmmmm

About 2 weeks ago, I began hearing a suspicious hum coming from the rear end (not my rear end, the truck's!) of my 99 5.9L. I originally thought it was a tire hum, but letting off of the accelerator causes the hum to go away. I am guessing something is amiss in the differential or driveshaft. Not being much of a serious mechanic, and not having a whole lot of extra $$$, what would you suggest?

About 2 weeks ago, I began hearing a suspicious hum coming from the rear end (not my rear end, the truck's!) of my 99 5.9L. I originally thought it was a tire hum, but letting off of the accelerator causes the hum to go away. I am guessing something is amiss in the differential or driveshaft. Not being much of a serious mechanic, and not having a whole lot of extra $$$, what would you suggest?

my 98 had a hmmmmm also and it happend when i let off the gas and then it progressed to constant but worse when i let the gas off. It was the rear pinion bearing. common on these trucks. a way to check that is crawl underneath the truck, grab the u-joint where it goes into the pumpkin, shake it like a mad-man and if it moves, its your bearing, if it doesn't, i'd take a look at your rearend gears, also common failure w/ these trucks. be sure to use a synthetic gear lube if you drop the pumpkin cover.

To take the rear end apart,you do need a special tool from Chrysler made by Miller tool.I do not remember the tool number and looks like a long 5/8'' hexbar with an attachment on it,it goes through the axle tube.The tool helps with setting the backlash.Before taking apart,check the backlash with a dial indicator and write that down somewhere.Inspect the rubber fill plug also,those are known for leaking

Thanks for the replies. I will do some more investigating on this. What kind of cost am I looking at if it is a bearing?

The bearing, crush sleeve, inner&outer races and seal cost me150-175.00. I dont remember. That was at a local auto store but out of the several stores we have here they were the only one who didn't have to order it. Also I was told to make sure its a quality bearing, i guess there are some crappy ones out there. I bought a Timkin I think thats how its spelled. I was told that was a quality one. I only put 3000 mi. since replacd but so far so good. You are SUPPOSE to use a special tool, but we got away w/out it. It pays to know proffesional mechanics. Either way the rear gears would come out so inspect the gears for flaking cuz i dont think you will wanna do this again if you dont have to.