"Buzzing" Starter

[Quadcylla]

I have been under my 2002 Explorer xlt 4x4 (4.0L V6) for over a week now trying to figure this thing out.
Here is what is happening:
When I attempt to start it, it turns over twice and then begins "buzzing" (starter pinion doesn't have enough power to turn and just "sticks"). A few days ago, I jumped it and it started right up. Tonight I tried to jump it and it just does the same thing... buzz.

Here is what I have done so far:
1) Read every thread on here I can about starters, batteries, alternators, and battery cables
2) Replaced the starter (Including solenoid)... twice. (I'm a pro now)
3) Tested the battery (it's fine - tested it at AutoZone and O'Reily on the same day, just to be safe)
4) Tested Alternator - tested good
5) The battery cables are crack and corrosion free (understanding that I cannot see the inside of the cables to be sure they are ok - but they look good)
6) Tested the starter relay - tested fine (according to the Haynes manual)

I'm at the end of my rope here. I need a car this week and I cannot figure this thing out. I would rather not try to tackle changing the battery cables (what a tangled mess that looks like it would be!) but if that's the general consensus, then I guess that's what I'll do.
Thanks for any help!

 

[Quadcylla]

Ok, just got done replacing the battery cables (OEM from Ford). It didn't work. Still just getting a "buzz" from the starter. Any ideas? (again, the batter has a full charge).

 

[jrowe6]

run a jumper from the batter to the starter to supplement the current flow to the starter. if that helps your main power line going to the starter is bad. dont forget to check your grounds.

 

[Quadcylla]

Did that before changing the entire battery cable wiring harness (Bought the OEM Ford cables which come complete from alternator, starter, A/C compressor, positive & negative cables and all grounds). Even when I jump it, it still buzzes at me.

 

[jrowe6]

any chance your engine is locked up?

you are 100% certain that starter is good?

 

[Quadcylla]

My first guess was the starter, so I changed it out with a remanufactured one... same results. So I took it back and got a brand new one, installed it... same results.
Got my refund for that one and re-installed the original one.
I'm pretty certain the engine isn't locked up because I was able to jump start it one time from another vehicle and it started right up. But I have not been able to jump start it since that one time. (but, that's where my mind is leading me right now. I may just bite the bullet and take it to a shop - ugh - and see if they can diagnose it)

 

[jrowe6]

i know you said you checked the grounds. just making sure....you checked the engine to the body/frame ground. and the ground from the body/frame to the battery's negative?