Any easier when the engine is off? Before you do anything, ensure that you have no tight bends in the cable. Follow it along as far as you can see. When it mounts to the tranny, make sure it's a straight run from the anchoring point to the lever. The cable also needs to move a bit in the last three feet or so; don't tie that section down or fasten anything to it. Disconnect the cable from the tranny; shift much easier or not much? If not, it's a bad cable. I have found that simply lubricating the end doesn't do much.
If you really can't buy another cable, here's an old tip that may keep you going for a while: you need to lubricate the inside cable through it's full length. You can't tell, but if the problem is simply a dry cable and not yet mechanical wear/damage internally, hear's what you do: Get a manual garden pressure sprayer or some way of forcing liquid though a hose. Clamp the hose over one end of the cable (remove rubber seal on cable if applicable), and force regular non mixed green antifreeze (that's right, antifreeze) through the cable. Learned this from an old sod that was a marine travel lift mechanic....works great.