Constant Noise seems to be from rear side of the car

It would need to be ran up on a lift, and listened to with a stethoscope. When is it loudest? When you accelerate, or whenever you let off the gas? Does it go away if you put the trans in neutral? Its possible the rear is worn badly considering the 268k miles on it. Has the fluid ever been changed, or the level checked?

Yes, while driving, put it in neutral, take the load off the rear, see if noise stays, or goes away. Just curious, why are you checking diff fluid level everyweek? Unless u have a known leak -like a pinion seal, it should be checked at every 5k. I wouldnt recommend just replacing yourbearings in the rear, as they are roller bearings, and mostlikely not even the cause of your noise. Most likely is a pinion bearing, at which point, you should have the whole diff gone through. Check for 6-10 thousandths freeplay from your pinion to ring gear. I usually set towards 6thousandths. But I think im getting ahead of myself hear. You need to verify exactly what is making noise.

Its the roller bearing that sits torwards the front of the differential. Your driveshaft yolk is pressed onto this pinion. This pinion sits inside that bearing.

Ok. If the noise is always there, then im certain its in the rear differential. You mentioned you didnt know much about cars, and were uncertain on checking the rear diff fluid level. Depending on how skinny you are, you may need to jack the vehicle up to gain access to underneath the rear axle. About 1/2 way up the differential, usually on the passenger side, will be an access plug. You may need to insert a 3/8" or 1/2" ratchet in to remove. This is where you check your fluid level. While your under there, check for any leaks. If its low, then it usually leaked out, or someone serviced the diff, and never filled it. Does this help?