If I had my truck worked on at a garage but the repair was not done correctly and caused damage to my vehicles drivetrain, are they liable?

You can sue the garage for negligence.  Negligence is the failure to exercise due care (that degree of care that a reasonable garage (repair shop) would have exercised under the same or similar circumstances to prevent foreseeable harm).

In order to prove negligence, you will need to prove duty (of due care mentioned above), breach of duty (failure to exercise due care by the garage's repairs damaging your truck), actual cause, proximate cause and damages.

Actual cause means but for the garage repairing your truck, would the drivetrain have been damaged?  If the answer is no, which appears to be the case, actual cause has been established.

Proximate cause means were there any unforeseeable, intervening acts which would relieve the garage of liability?  If the answer is no, proximate cause has been established.

Damages means the amount of compensation you are seeking in your lawsuit for negligence.  Your damages will be the cost of repairs to your truck.  You will need to mitigate (minimize) damages by selecting a repair shop whose charges are comparable to other repair shops in the area.  If you were to select the most expensive repair shop you could find, your damages would be reduced accordingly.  If you need a rental car while your truck is being repaired, you will need to mitigate damages by selecting a rental car with a reasonable rate.  If you were to select the most expensive rental car you could find, your damages would be reduced accordingly.

Depending on the amount of damages (cost of repairs to your truck plus rental car if applicable), you might be able to file your lawsuit in Small Claims Court.  Your damages should also include court costs which would be the court filing fee and process server fee.