i've got an 85 T/A with the "spin resistant" tag on it, Its got 10 bolts on the cover, it looks similar to the 10 bolt my S10 had but its got a rubber plug on the cover and the bubble on the left side sticks out more? it seems to be original as well, anyways... when i'm going over 40 it makes more and more noise as i speed up and it seems much louder with the windows closed

the diff seems to lock up, it puts a bit of a drag on the engine when i'm turning over 20mph and barely touching the gas but it seemed to lock up much harder at the beginning of the summer (i got a little wheel hop over 40mph on on-ramps now i don't get any regardless of speed) it makes the noise the same whether i'm going strait or curving on the highway (700r4 with 16X10 rims 1500rpm's at 50mph)

I'm wondering if I just need to replace the fluid or if my diff is now open

 

Count the bolts again. This is the 9 bolt Borg Warner rear end. You might have a bearing or two going bad, or the gears might be going bad. You can try changing the oil. That's a cheap thing to do compared to rebuilding the rear end. Maybe it will help. Don't forget the GM limited slip oil additive.

 

alright i'll give it a try, I'm assuming I need to take the cover off to drain the fluid? and do I need to go to a dealership to get the additive?

 

Yes and yes.