Need diagnosis for sudden 5 sp. man. trans. problem

[camaroincal]

Car is a 1988 Mustang lx 5.0 Basically, my car was running pretty good up until I was on the highway going about 70 mph...I dropped it into 3rd from 5th and floored it until redline..over 100 mph. I then took it out of gear and tried to put it in 5th from 3rd..Wouldn't go. Came to a stop, and had a hell of a time getting it back in 1st. Now, when I come to a stop at idle, the car vibrates/shakes which it didn't used to. It is also now always difficult to shift into any gear.

Also, when taking off normally from first gear now, the car bucks more than it used to. It always has a little since it has 2.73 gears, but now it's more pronounced. Also while cruising giving it light gas, it starts to buck lightly and randomly. I had felt this before and thought it was a slight missfire, but now it does it even more. Just wondering what this most likely could be. Any help is greatly appreciated.

Of note, I just got the car 6 months ago and it has about 157,000 miles on it, of which I've only put on about 1,500. For the drivetrain (clutch, tranny etc.) I don't know what's stock or not...The clutch peddle has always been a bit stiff but I've never had problems shifting. I always thought maybe it had an aftermarket clutch.

[KimMG]

Tune-up
check all the fluid levels