alternator change and still wont start

no power ...check both cables and grounds........

If it starts with a jump then something is wrong at the battery. Did you replace the battery at the same time as the alternator or was it new until you realized it needed the alternator? Like Kelly asked are both the battery cable ends in good shape, if not then clean or replace them. Has the battery been checked since the alternator was replaced? Is one of the battery post plastic caps still on it preventing contact with the cable end?

both cables are replaced and the battery was replaced when I got the alternator. how do I check the grounds?

check your starter.

Clean the battery terminals even if they look clean. When trying to crank it measure the battery voltage. If it is under about 10 when trying to crank either the starter is drawing too much current or the battery is bad. An addition battery during a jumpstart will sometimes provide enough power to turn a defective starter.

As I understand what you've described by jumping it off it starts and runs. If you are jumping it at the pos and neg post of the battery it's as if you are putting another battery in the truck for a moment. If you still suspect a ground issue just follow the neg cable to wherever its bolted to the frame and engine then clean and tighten as needed.