Did you triple check all your ground connections and try re-attaching them?
Bad grounds will cause all sorts of weird issues.
I.E. Car with a broken ground strap would start slow then all the dash lights turn on & off and lose the speedo but the car still ran.
My 03' after swapping in the LQ9 did just what your saying, the battery gauge showed 12volts and so did the battery test too. Hit the starter and it would drop down. It took me 3 times to get the dam ground strap to make a solid connection.

Toss that other starter in there and if it still does the same thing it's definitely a bad connection somewhere.

 

HOW DO YOU GET THE STARTER IN???

Does it go in with all the wires attached or not? I got it in with nothing attached but then I had no room to attach the wires.

edit: finally got the starter in, but it seems I have a major electrical problem.

When I first rehooked the battery and went it and turned it to the one position everything was normal. All proper dash lights light up, radio worked etc. But I turned the key to start it and nothing, dash went all crazy and the turn signals came on, lost radio, it's like I lost all battery power.

 

I put the wires on before I swapped the starter. I had the same issue you are having. I found it to be a bad ground. I had to remake the ground from the battery to the block. Once I did that I have had good luck. Double check all your grounds. Funky stuff starts happening when one is bad. I replaced the starter, battery, and alternator and then found it to be a grounding problem.

 

If I remember correctly I put it in without the wires attached and hooked them up after wards. I think I hooked up the wires through the fender well after removing the plastic cover.
Sorry don't have a LSX truck anymore to take a quick look at it.

 

Exactly, start with the cheaper option first like cables then work your way up the the expensive parts last.

 

I GOT IT I GOT IT I GOT WOOOOOOO

Bad ground on the block guys. It starts like a CHARM now. I can't tell you guys how relieved I am, tired and messy, and I really want to thank you all.

I just can't believe a poor ground could cause that weird of problem

 

Dude that awesome, glad we could help!

Now go burn off those tires!

 

I'm already thinking about mods lol. I just went and drove around and guys I have to be honest between 500 and 2000 rpm the 6.0 doesn't have the torque I thought it would, it's honestly not a huge change over the 4.8. Maybe something's wrong I don't know. Maybe I should go get a baseline dyno

 

could I just confirm that a 2000 4.8 and a 2002 LQ9 use the same injectors?

 

I know it somewhere on the forum if you search but I'm sure the 4.8 used 24lb and the 6.0 were 26lb. Not enought to make any difference because even 24lb injectors will be enough for a 6.0 until you start adding mods like cam, heads, etc.

I might have missed it but is the LQ9 stock or modded? I'm sure it's just in the tune.
I know my LQ9 felt way underpowered compared to the 5.3 when I first put it in but a good tune made all the difference in the world.