its not that difficult to remove, but maybe that is because i've done it a few times. while it is possible to remove individual instruments the speedo and clock are probably the most difficult to remove on its own, due to its location - the console and the steering column tend to get in the way. you can pull the wooden panel out the front, which is perhaps the easiest way, but i find it easier if you remove the steering wheel.
to remove the entire cluster, you need to loosen the nuts (under the dash) on either side of the dash instrument surround - don't remove it, just loosen the nuts. you have to then loosen the cluster wood panel (its been a while so i forget what you have to do). then push the cluster forward on one side so you can then access the speedo cable and unplug the cluster wiring plug (on top of the steering column). once removed its easy to remove the wiring (mark where they go) and pull the light bulbs out and unscrew the gauge fasteners. then you can send the gauges off to get them cleaned and working - have the speedo gear replaced while they are in there. there are several places listed in the FAQ - Best Sources
you might also check the speedo cable ... but i suspect its more of the instrument itself