W124 Hard Shifts

[350SL4spd]

Hello all,
My '91 300E has been shifting relatively hard for a long time now. I have been trying to ignore it (too many projects and I am overly sensitive about these things anyway) but now I have a little free time... Fluid is full, fresh filter. Usually 3-4 is harder than the rest (sometimes it's 2-3, and yes I understand the 2nd gear start, it's definitely 2-3 sometimes), but all are harder than the '91 300TE we have. Not really bad, but noticable. The thing is, it seems to be in the back end...which makes me question the rear flex/diff. I have checked the rear flex disc, and it will need to be replaced in the near future, but it certainly doesn't look bad enough to cause the hard shifts.
Could the diff fluid be low? Is my tranny going? What else might be the prob? I much prefer to find these things before they get too out of control. If it is the tranny, diff or both, that wouldn't be too bad, as long as they give me a little while longer...(at least until I find my donor car for my manual conversion/ or a limited slip diff .)

Thanks for any insight .
Cheers,
-Matt-