Could it be a problem with the gear shifter itself?
Rather than type all this out and sound like I know
what I'm blabbing about, I copied this from the Factory
service manual:
HARD SHIFTING
Hard shifting is usually caused by a low lubricant
level, improper, or contaminated lubricants. The consequence
of using non–recommended lubricants is
noise, excessive wear, internal bind, and hard shifting.
Substantial lubricant leaks can result in gear,
shift rail, synchro, and bearing damage. If a leak
goes undetected for an extended period, the first indications
of component damage are usually hard shifting
and noise.
Component damage, incorrect clutch adjustment,
or a damaged clutch pressure plate or disc are additional
probable causes of increased shift effort. Incorrect
adjustment or a worn/damaged pressure plate or
disc can cause incorrect release. If the clutch problem
is advanced, gear clash during shifts can result.
Worn or damaged synchro rings can cause gear clash
when shifting into any forward gear. In some new or
rebuilt transmissions, new synchro rings may tend to
stick slightly causing hard or noisy shifts. In most
cases, this condition will decline as the rings wear-in.
TRANSMISSION NOISE
Most manual transmissions make some noise during
normal operation. Rotating gears generate a mild
whine that is audible, but generally only at extreme
speeds.
Severe, highly audible transmission noise is generally
the initial indicator of a lubricant problem.
Insufficient, improper, or contaminated lubricant will
promote rapid wear of gears, synchros, shift rails,
forks and bearings. The overheating caused by a
lubricant problem, can also lead to gear and bearing
damage.