So, I went and worked on the tractor again. I drained all the oil out. Removed the internal PTO shaft by pulling it out of the back of the tractor again. Removed the access panel on the left side where the PTO shift lever is. Checked all the teeth on the PTO shaft, no wear on it, looks perfectly fine. Checked the teeth inside where the end of the PTO shaft slides into, no wear, looks perfectly fine.
Once the PTO shaft was out, I held the clutch down, then I reached inside the tractor and could manually push the rod (that the PTO shift lever pushes on to engage and disengage it) forward to engage the PTO. Just like when I did it the first time, it went forward to the engaged position.
I put the PTO shaft back in from the back, with the PTO still in the engaged position, and put the four bolts back in. I put the side access panel with the PTO shift lever back on and put the bolts back in. Then I put the oil back in.
Once it was all back together, I didn't have any problems moving the PTO shift lever back and forth from the engaged position to the disengaged position. So, I started it up to see if it would give me problems again. Even though this is exactly what I did initially to try and fix it when I couldn't get the PTO to engage.
Now it will engage and disengage without any problems or grinding. I hooked up the mower deck, drove out into the field, and cut the last 2 acres that i had to do without any problems at all! Everything is working fine again.
I don't know what the problem was to begin with, or why it wouldn't work once i hooked up the mower deck last weekend, especially when all I did this time was exactly what i did the 1st time to try and fix it????????
But I got it working now and haven't had any more problems.
So, I'm glad it's fixed, but still completely confused about the whole situation. And I want to say Thank you to everybody that posted and helped me with this problem. I really appreciate everything!!