Its known as a "midship bearing" in case you ever have to get one, they are a bitch to track down, not something the local auto zone would have ,, Although they may be able to special order it.
I'm hesitating to say that could be your problem, they are supposed to have very little, if any, play.
I completely wrecked mine, along with the trans support cross member, going over a bump a bit too big.
This was last year, I'll check mine later today and see if mine has any play.

Don't overlook tires as a potential cause of rumble, a failing belt will produce those exact symptoms at certain speeds, just as an imbalanced/defective drive train.
Best to jack the front up, and spin the tires and see if anything is obvious ,,, Though sometimes the defect is so subtle that only a pro tire/alignment guy can spot it.
Could also swap the front to rears, if the problem remains, more than likely not tires.

 

Never heard it called midship bearing. Autozone does stock them. They list them as center support bearings.

 

have u changed any fluids in the rearend cause u said ur hearing a noise when u turn, i didnt know it til someone told me on here but the rearend lube requires a special additive. i also kinda had rumble and othe noise i had to replace my pinion bearings

 

xray99 is dead on about checking/rotating your tires first just to see. My Explorer had the same problem, at certain speeds, it made a grinding, roaring noise that I was told by 3 different shops (I was out of town) was possibly the front half shaft (which includes the wheel bearings $$$) or the front diff. Took it to my personal mechanic when I got back, (he's honest) he checked over everything, and then rotated the tires. That was the problem, it now runs quiet as a church mouse, well so-to-speak. So I learned my lesson about tire rotation!

 

Haven't changed any fluids. Checked the level and it was okay.
Inside the glove box it lists the truck to have an LSD but if you turn the wheels they spin opposite of each other when off the ground which I thought meant an open differential.

 

I know the tires need replacing and that may in fact be the cause of the rumble but I really want to figure out that noise in the back because if I have to rebuild the rear-end the tires will have to wait for a bit.

 

mine does to more than likely u have the Trac-Lok diff i would try changing the fluid in the rearend and get the special trac-lok additive. The trac-lok diff has clutch disc and they need a spec additive, mine did the same when i was turning corners. have u cracked open the rearend at all? i been playing hell with my rearend had to change the pinion bearings, now im hearing ring gear noise i didnt get the pre load set right, i also know i have a bad thrust washer. but when i changed fluid i put the 140 in and i havent heard noise when i turn corners so again i would try changing the fluid first.

 

Hi. No I didn't change the fluid or crack the cover off yet. I didn't want to waste the time/money on refilling it if I knew for a fact I had pinion/gear or bearing problems. Can you buy the LSD additive in parts stores or do you have to get it from a dealer?

 

You can get it at most auto parts stores. You need 8oz for these trucks. Sounds like the clutches may already be gone if you turn a wheel and the opposite goes the other direction.