When I'm driving in 4th or 5th gear, at a specific rpm range, the wheel vibrates and I get an odd feeling in the gas pedal. It happens when I'm pushing the gas pedal about half way down. Feels like I'm putting strain on something (engine or tranny). I can hear a deep sound.

Happens exactly at 3400 rpm in 5th gear at about 80mph every time. What could this be? If I let off the gas and slow down a bit or floor it, it'll go away until I reach that point again.

This is kind of iffy, but I'd start off with a rotate & balance and an alignment. See if that helps, and if it doesn't, then at least you've got that knocked out.

-Roman

Doubtful, just got a four wheel alignment and balance not long ago with new tires.

Hit any pot holes recently?? That would definately throw off your alignment and possibly even throw a wheel out of balance.
You should check your front hub assemblies as well, to make sure they turn smoothly. I'd eliminate all possible mechanical defects outside your transmission before tearing it out looking for something that might be elsewhere. Check also your motor mounts I'm not real sure what weird feeling you are getting from your gas pedal. Basically just give your car a very good once over and have the wheels checked for balance. It's very possible you lost a wheel weight while driving down the road minding your own buisness. It never hurts to check it out before you go tearing into things.

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95 Eclipse RS - Rolled
96 Eclipse GS - Traded
95 Eclipse GSX- Sold
01 Blazer LS 4.3/5spd dropped and clean DD
88 Camaro RS 361 CID with a 6spd and a bar 10 second street car on motor.

OK, I noticed this problem occurs in every gear at exactly 3500 rpm. It's weird because once I exceed or go under 3500 rmp, it goes away. Would that eliminate the transmission from being the culprit? I rotated the tires, didn't help. Will check the balance this week.

I don't know, but on the highway (not condoning this action either), but at 90 mph the car shakes, but once I hit 95 it gets perfectly smooth.

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-Ryan

Tighten the control arms.

Here's my thought:
It sounds like you've found your car's resonant/natural frequency. Every system has one and the most well known exploatation of it is when a wine glass breaks because of the woman singing. Basically, in your case, you probably have an unbalanced rotating assembly either with the crankshaft or the main input shaft of the trans. I'm not really sure which one but my guess would be with the crank. Someone else probably knows which venue to seek better than I but there's probably a bearing which is failing and allowing excess oscillations.