The way that I always check the condition of the linkage and shock bearings is to place the bike on a stand, rear wheel off the ground, then pull up on the swingarm near the rear axel. If there is play, this would indicate that the bearings are worn. Additionally the bearings can be seized and rusted bacause a seal is blown, to check this you will need to strip the linkage. Another issue that will cause the rear of the bike to ride low is the suspension sag setting, this maybe adjusted by turning the ring at the top of the spring clock wise. Do you have a service manual?