So I recently bought a 95 SR with lots of miles on the clock. When I bought it, it had a noticeable vibration over 45mph. I figured it was a bad tire or just needed a balancing. Well after replacing tires and wheels and having them balanced I am still getting the vibration.
It appears to be coming from the rear driver's side. It is also accompanied by a wah-wah like humming noise that inceases with speed. Do you guys think it is possible I have a bent axle shaft or a bad bearing?
Any Ideas?
I'm tempted to put it on jackstands and put it in drive and watch the rear wheels. Do you guys think this is safe?

Tire or wheel could be out of round. Try moving it to another axle to confirm.

It could also be a bad shock allowing a wheel/tire to oscillate.
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A bent axle shaft is highly unlikely. A bad wheel bearing could make the noise that you're hearing but wouldn't create any vibration. I'd be looking at your driveshaft u-joints as the first and most likely culprit. Do you have any pulsation in the brake pedal when stopping? A seriously warped brake rotor could create vibrations that you'd be able to feel while driving as well as when braking.

If you're going to check it while it's up in the air and running, make doubly sure that you have it securely supported with your jackstands under the frame - I would leave the front wheels on the ground and chock them securely. Take the rear wheels off and have someone run it at the offending speed while you look and feel what it's doing without getting underneath it.

If you don't feel the vibration while running it with the wheels off the vehicle, you've probably got an out-of-round tire. Tire shops would love to have you think that when it's been balanced, its going to run smoothly. That's not the case, you could balance a square tire if you had to but it doesn't mean that it'll ride smoothly. You might need to have the tire road-force balanced, which measures the force that the tire puts against a roller and can detect an out-of-round tire.

Ditto the ujoints

Did it with both the old and new wheels and tires. So not impossible but I'd call it highly unlikely. Just rotated them today and it's still in the rear.

Also has brand new KYB Mono-Max shocks

My bet is on u-joints.

So basically if the if the U-joints are worn, the driveshaft no longer spins around its own "center"? I could imagine that would vibrate quite a bit.

So whats the process for checking u-joints? A pry-bar between each side of the u-joint?

Also there seems to be a two different part numbers for 95 rear U joints, precision 412 and precision 450.

Park it, put it in N, set the ebrake. Crawl under and wiggle the driveshaft while looking at the ujoints. If it's vibrating, you'll see and feel the slop in the joint. There will be some backlash in the rear end gears, so don't let that fool you.

There is little bit of slop, but it does not seem extreme. I can imagine it move around alot more under torque from the engine. It's significantly less than the amount of backlash in the rear end gears.
Think it could be the issue? ideally A driveshaft should not have any play at all right?