Transmission mounts must be visually inspected to determine if they have cracks, rotted rubber or complete breaks, all of which require replacement. Other issues such as loose brackets or missing bolts can be easily repaired. The transmission can be heard banging against the mounts if there is a problem.
To determine whether a transmission mounts needs to be repaired, crawl underneath the vehicle with a flashlight, examine the mounts and identify any damaged mounts. A loose or bad transmission mount must be fixed or replaced immediately, as movement can cause severe damage to the transmission's interior mechanism. This damage may eventually cause transmission failure, which is difficult and expensive to repair. Transmission mounts themselves are cheap and quick to repair.
If you find a damaged mount that must be replaced, jack up both the front and back tires of the vehicle and place on jack stands with the tires at least three inches off of the ground. Place an additional floor jack under the transmission oil pan to take the pressure off of the mounts. From this position, remove the damaged transmission mounts. You may need to use penetrating oil to break the bolts on the mounts loose due to rust and road grime build up. Replace the damaged mounts and lower the vehicle.