Do you have a voltmeter/multi-meter/amp clamp??   If so, test the starting circuit see where you do not have the proper voltage or where you are drawing too many amps.

Your statement that "if we hook up to the solenoid now with a 100 amp jump, it will VERY slowly rotate"  makes me think you have ruined the starter.  The multi-meter and amp clamp will verify if the starter is indeed bad.

I cannot tell from your post if you have changed the starter, the solenoid, or both. Assuming you have indeed changed the starter and it will only turn very slow with a 100 amp boost, the starter may be the wrong one, engagement of the teeth may be too tight, or the new starter may be "bad".

Griff

Well from an old hand in the mechanic world troubleshooting everything from Cadillacs to Cats and all in between I'd go with what Griff said. Check your draw while cranking. You said you hooked your leads with your machine and cranked but no fire. I'm assuming that it indeed did turn over? If it did turn over, at what would be normal cranking speed while starting then you could have a fuel issue. Pretty much all of the diesels I have ever touched, construction wise has had a shut off solenoid for the fuel rack. Energized pulls the fuel rack one way, allows fuel to flow to injectors and baboom, it starts. De-energized the rack kicks back blocking fuel and it stops(or visa/versa can't remember but if I was standing next to it, no problem). I have replaced a few of these on Deutz and Kubota diesels in my time. They'll crank all day long and won't hit a lick. An old timey method is to shoot a bit of ether in the intake while cranking, very important to do WHILE Cranking or you could hydro lock the motor. If it fires off and tries to run then you have your fuel issue. If the fuel shut off solenoid is working, can check by having someone kick the key on and feel/listen to the solenoid, it should make an audible click or it may have a rod attached and you'll see it move.

If that works, cranks, solenoid is good and cranks all day long may have a fuel pump issue. Most of the newer stuff has electric fuel pumps. I've even seen some that had a small screen/filter in the outlet of the fuel pump that would get stopped up and you would have insufficient fuel flow to feed the pig, ahhh, the Takeuchi track type skid steers had these things. We ran around for a month or more trying to figure one of these things out, nothing in the maintenance manual about this screen. If you have fuel flow, usually they have a bleeder on the fuel filter. I'd usually crack these(field service mechanic) out in the field and turn the key on to energize the pump and watch for the fuel to spray. Then I'd work up to the injection pump and crack the lines(nuts) on top and crank it over to see if I got fuel out, if so I'd go to the injectors and do the same. Sometimes they would even fire up and run without even a hiccup.....usually would find out some dimwitt ran it out of fuel but did not want to get hit with the "idiot" fee from the rental company for the service call.

If it does not crank over, do what Griff said. Check the voltage drop, put the neg lead from your dvom on the neg of battery, positive to positive and crank over....or try to. The voltage should not drop below 10v's, pretty much the norm on everything I have ever touched. If it does you can try a few more times and put your hands on the positive or negative cables, check to see if one feels warm/hot. Could mean a problem with the starter or corroded pos/neg cables, back connections or even bad grounds. I have seen a dirty ground make the dash on a Cadillac do some freaky crap. You could ohm your cables out, should read about .2 ohms, higher than that and you could have corrosion, damaged cables and so on. Starter solenoid, dvom ground to battery, positive to battery side should have 12v's, go to other side(starter side) have someone crank over...or try, again, should have 12 volts. If the solenoid clicks, pops 12v's on your meter test on the positive terminal of starter, if you have 12v's pop starter off, drop it on the ground hook your positive jumper cable to the positive post, ground to the body and a small jumper wire to go from positive to small post to actuate the bendix, it should kick out and spin if it's good, if it just clicks or does doodly then drop it by your local starter/alternator repair joint probably has a problem with the armature, brushes etc.

Most of the time I could get out there, try to crank it and pick my direction, electrical or fuel. Electrical, I'd jump straight to the starter with my test light, hit the key and if the test light came on at the solenoid on the starter, I'd pop it off and "bench test" throw in truck and be gone. Where most of you guys are in welding, I'm at as a mechanic(just don't pay worth a hoot). After 18 years I finally realized, ..... I'm good. Sorry, can't be humble about that.

Good luck, if you have any questions shoot me an email would be glad to help. May take me a few minutes to dig the stuff out of the cobwebs in my head but it's all up there.....somewhere!