JUST PUT IN A NEW STARTER . DRAINED 2, BATTERIES. WHAT SHOULD I CHECK?

starter motor or starter relay
have an auto electrician check the system out
Why 2 batteries as the car only needs one

Maybe you have a short in the ignition switch. Put the battery back on, and everything connected EXCEPT the small red wire to the starter relay. Key off, of course. Now connect the red wire to the relay: if this causes the starter to engage, you probably need a new ignition switch.

Have you tried pushing the lever like you are lowering the bucket while starting it? Sometimes the pumps on these can drag the engine back while starting, especially if cold. It's best to shut them down with the bucket in the air some. Let me know how it goes.

On a level surface, chock the wheels,apply parking brake
remove keys and put in pocketDisconnect batteries,remove wires and tag them so you know
where they go when you put them back on, unbolt the 3 starter
bolts, usually 2 are easy to get at and one requires using a long
extension on a ratchet. once the starter is out, examine the teeth
on the ring gear that is exposed when the starter is removed to make sure that the teeth aren't damaged.If they are ok then install
the new starter, put the bolts in a couple of turns to support it, the wiggle it around until it drops into place. tighten the bolts equally
and reconnect the wires and tighten their connections, especially
the large positive and negative battery cables. If connected/mounted properly. and the batteries fully charged,,the new starter should
work immediately when they key switch is activated

Possibly starter bad, bad distributor shaft or electrical short in vehicle. check connections. If getting drained when sitting, check dome light switch or starter.

Sounds to be the Starter or starter solenoid .

Check your positive terminal for corrosion. Older GM terminals often have some built up inside the rubber boot that cant be seen from the outside. This in turn increases the resistance of the electrical circuit. (Not enough current is getting to the starter.) I would replace the terminal bolt also if it rounded off or if the threads are damaged. They are only 2 or 3 dollars.

Use a really small flat head screwdriver to pry it out of the boot then you will be to check the inside of the boot.

Some GM's will have 2 separate positive cables. If this is the case, the cable without the bolt will have a metal spacer inside the boot that can be removed for cleaning. The bolts come in 2 sizes. Short for one cable, Long for two cables.

there is more to a starting problem than batteries and testing the starter. the large share or starting problems are the following. bad cables, bad grounds,bad batteries starter bushings in the starter bad windings and brushes . first do a load test on batteries or as in this case battery replacement . were they fully charged. just because there new they need a full charge. second check cables and ends just because they look good on the out side doesn;t mean they are good they can corrode on the inside and the result is resistance cutting amps to the starter.third is the starter some will test good out of the vehicle but put them under a load and they will bind up and drag which will slow them down I will put money on the starter drag and if you need to save money try ebay a seller call bp electric sells a great one for about 89.00 plus shipping compared to 300.00 at the auto store and they work great I have one on my 94 3500 three years now.

your have a wire shorting out, check the hot wire to the starter you replaced, chances are there is a bare spot on the wire and is grounding against the motor block

No sir, a load tester does not give an accurate diagnoses, of a batteries condition. The Specific Gravity of -> Each cell, has to be checked also.
Believe it's 12.70, but don't quote me. I suggest with a good battery, that is fully charged, try again.

Also note that a battery in a drained condition can make an alternator go bad. The alternator works too hard. Batteries need to be fully charged, then put in the vehicle.