Manual Valve Body Hard Shifting.

I had a simular problem once, and it wasn't hard to shift hard all the time. It turned out to be the trans mount. Seems the trans would move just enough during left and right hand turns, that the linkage would rub against the cross member, and bind. Sitting still, the linkage always seemed to shift smoothly. I replaced the trans mount, bolted it down tight, and problem was fixed. Check your motor mounts, too. Even if all mounts are good, check to be sure your shifter bolts are tight, and the linkage has plenty of clearence.