So far, I've replaced the glow plugs with new Motorcraft, installed two new batteries, and I've replaced positive battery cable harness with a new one. All this helped the cold startup problem, but I still have a warm startup problem after the truck sits for about 15 minutes or so.
On a cold startup, I can usually get the truck started in under a minute whereas it sometimes took over 5 minutes before I changed the glow plugs. However, once everything gets warmed up, its starts ok as long as it doesn't sit for more than 10 minutes. After about 10 minutes or so, I have to spin and spin the engine before it will crank. Usually it has to spin for a good 10-15 seconds before it will crank on a warm startup. Whats weird is that the starter will turn over kinda slow, then, the closer the engine gets to cranking, the starter will speed up. There is no miss or skip before it cranks that helps the starter kick the engine over. The starter just starts off spinning kinda slow, gets faster and faster, then the engine fires up.
I live in a fairly warm climate this time of year so I shouldn't be having these cold or warm startup issues. I believe on a cold, and especially a warm startup, that I should be able to crank the engine as soon as the 'Wait To Start' light goes out without all the continuous starter spinning.

What else could be the problem?

 

I would like to recommend you have the starter checked out. If you can count the engine RPMs on a cold start it's too slow. On a warm start it must go twice as fast.

Where is the fuel pedal during your starts....full down, 1/2 down or resting on the fast idle (cold) or low idle stop (warm).

 

Its not quite slow enough to count, but its close. On a warm start, it starts off spinning about the same speed as when cold, but it speeds up really fast after about 10 seconds of spinning. If the truck sits for around 10 minutes or less, it will fire right up like its supposed to. No slow spin or any other problem.

When cold, I usually hold it about half down. When warm, I don't touch it.

 

I would be willing to bet that your starter brushes are worn down to the point that the tension springs are resting on the brush holders, and there is very poor contact pressure between the brushes and commutator. I had the same problem, had a electrical repair shop replace the brushes and turned the starter commutator for $50.00. Starts good as new

 

I agree. The starter should spin fast from go.