Why won't my car start? Battery seems weak. Had it checked. Only 2 yrs. old.

Sure you can tap it, hard. Then check soleniod and wire ends an well as engine ground. Starter is most common as you already know.
Most starters from auto parts stores are 'remans'. Some are better than others.

your missing ground wire . run a piece of # 2 wire from engine block to were your battery ground is located to the body of the car

starter cannot act good if it does not spin the engine over quickly with new battery
pull it for a bench test under load
check and clean terminals and check for good ground point

Might be a weak battery.

Check the battery like this:
Turn on the headlights, then try to start the car.
If the headlights do not dim or go out, then the battery is likely ok.
Check that the battery cable plus post, goes to the starter, Minus post to a good ground, like the engine.
If wiring is ok, then look at the starter solenoid for a good connection.
Problem is most likely the Solenoid or starting relay, whichever the car uses.
Hope this helps.

Check all ground wires and make sure they are tight. Start at the battery, starter, solenoid switch.

SOUNDS LIKE LOOSE WIRES AT STARTER OR GROUNDS TO ENGINE BLOCK
ALSO MAKE SURE BATTERY CONECTION ARE TIGHT AND CLEAN
IF ALL THIS CHECKS OUT HAVE STARTER BENCH TESTED

On the firewall is the starter motor relay. There are 2 large battery sized cables to it, and 1 smaller ( 16 AWG ) wire that is orange or orange with a stripe ( forget the color, want to say white or tan ).

When you turn the key to the start position, check the smaller gauge wire for +12 V. The case should be ground. If you do not get +12 on the orange wire, try starting the truck in neutral ( apply the parking brake, chock the wheel, etc normal safety precautions ). Test for +12 V at the small wire. This is the output of the DTR ( Digital Transmission Range ) sensor or in common terms the Neutral switch. The start position on the ign switch runs through the DTR to the starter motor relay coil.
If you have a stick, the DTR is replaced by the Clutch Pedal Position Switch ( clutch must be pushed in to start ).
If you have power to this ( at the times the truck won't start ) next is to check the both battery cable ends to see if they are corroded and making intermittent contact ( not probable but possible ).

In order of probability:
1. Weak CURRENT level on battery (may show good voltage but no CCA left).
2. Bad/poor connect at terminals or ground wire on frame or battery wire on starter.
3. Starter - jump start with another vehicle - if that works, starter is fine.

not familiar with immobiliser. i would check connections between ignition switch and starter relay and connections between starter relay and starter, in other words that whole starter circuit. some of the connections may be weak where they go to power or ground. new battery makes it work okay, then it after while not so good makes me suspect those connections. it could also be a starter problem, might be a burnt place on the starter, when the starter motor stops in a certain place it can't get good connection and so won't let current pass, motor can't turn, no start. hope this helps.

Could be your neutral safety switch, or it's not all the way in park. Broken or loose wires in the starting circuit. Inspect all wires and connectors at the battery, starter solenoid, and ignition switch. Or your starter pinion is jammed in the ring gear. Remove starter and inspect. Last but not least, your engine is seized. You don't want that one. Good luck.