Nearly fired following doc's orders. Wrong?

I was threatened to be fired from my job of 4 1/2 years as a "P&D"(pick-up and delivery) truck driver by my dispatcher because I told him in a nice way that I needed to come back in to get ready to leave for home because I had to start preping for a colonoscopy I was having this past Friday. About 2 weeks prior I had suffered a flu like illness and internal bleeding. Luckily it only lasted a few days but I went to the doctor to see what I needed to do to find out what was wrong, which resulted in a scheduled colonoscopy. I let my employer know 1 1/2 weeks in advance of needing off for the proccedure this past friday. Unfortunately, I only gave 3 days notice for a 1/2 day off this thursday because I had planned on working all that day because I didnt completely understand earlier about the necessity of needing to be home while taking the medication to prep. I called my doctor back to make sure of what time I needed to be home Thursday to prep and was told that I MUST be home no later than 4pm to start taking the medication and that I could not take it at work.
I was told the 1/2 day was ok by our regional manager who was filing in for our terminal manager this past week. The regional manager said to let our dispatcher know. I did and he basically said he would not guarantee me being able to leave at any time. Even when I told him our regional manager said yes he pretty much said he didnt care and still wouldnt guarantee anything. I then told him that if thats the case then I would get with our dock supervisor and have him set my delivery route close to the terminal so I could get through early, hopefully make everyone happy, and be able to leave early with no problems. The dispatcher said "ok". I know the best thing to do would have been to go right back into the office and talk with the regional manager about the situation but I had so many worries on my mind that I just walked out, got in the comapny truck, and started my day. That Thursday I came in at regular time which was 9am and finished my route faster than normal and made 3 of the 5 pickups(customers shipping freight by us) assigned to me. I then called the dispatcher at 2pm to let him know that everything was taken care of other than the 2 pickups that did not close till 5pm, which he had plenty of time to take care of as there were other drivers close by and more that would be coming in to the terminal before 5pm. He informed me immediately that I "would NOT be coming in to go home no matter what the reason and that was the end of the discussion!"
Unfortunately it turned into an arguement. I informed him again that the regional manager had said it was ok and of how important it was that I be home in time enough to start the prep for the test because otherwise, if I did not get home and take the medicine and give it enough time to work that I ran the risk of the doctor stopping in the middle of the test that Friday because I had not done what I was instructed to do. The dispatcher basically told me I would be fired if I came in and left for home and that he didnt care who said I could leave. I asked to speak to the regional manager and was told he wasnt there. I called our Human Resources dept and went through the long process of trying to get something done so I could leave without getting fired. Thankfully the person in our HR dept was understanding, but he still had to follow proceedures and said the dispatcher could fire me if I left unless he could get the Regional Manager on the phone to solve the situation. He tried to contact the RM but was unsuccessful. He then called the dispatcher and tried to work with him to resolve the issue. The dispatcher basically tried to play doctor and tell the HR person that he had the same proceedure and was at work all day on the day he had to prep and gave a lot of excuses of why I did not need to leave. He then proceeds to tell the HR person that he had talked to the Regional Manager after I left the day I had requested to be off the 1/2 day and had the disagreement with him(the dispatcher) and that the RM told him to "Do what he needed to do".. Ofcourse that was the first that I had heard of that.
The HR person was able to find out from my doctor after I gave him permission to, that I could start taking the medication to prep as late as 7pm that night since I was in the situation I was with my employer. This made me feel like a fool then but I told the HR person that was fine and that I would go along with that to keep my job and still, hopefully not have any complications. The trade off to starting so late would be the risk of the medicine not having time to work and that I would be up all night. After not being able to eat for nearly 15hrs by that time and knowing I had to stay up, when I could have slept through some of it, another 18hrs with nothing to eat was not something I wanted to have to do but I had to anyway to keep my job.
The really sad fact of this whole matter is that there have been numerous time that other employees, or myself, have had TRUE emergencies, medical situations, or just needed a short time off like a couple of hours and have been threaten or turned down by our dispatcher and/or our terminal manager(who is worse than the dispatcher) for such requests.
One small example. An empoyee who is no longer with us had a family emergency and called in to let the dispatcher and terminal manager know that he had to leave and was told that he could not leave under any circumstances. The employee's wife had been in a car wreck and had been taken to the emergency room in serious condition and the employee was told he couldnt leave!!!!

Hopefully someone out there can give me a detailed answer as to whether if I would have left work anyway when I origionally was instructed to by my doctor that Thursday, with all the details that I have provided and the examples of how myself and other employees have been treated in the past, of whether my supervisor would have had a legal right to terminate me.

I live and work in Birmingham, Alabama and the corporate headquarters of my employer is in Willmington, Ohio.

Thanks in advance for any help.
Sandman