Hi, Joey - Thank you for your answer!
An update: As I was unloading the wet towels from the washer, when I had almost finished, as I lifted out a handful of towels, the washtub kind of "popped up" - making me wonder if somehow the weight of the load had caused the problem (even though it was not an overstuffed load of towels, and I am certain I had washed that number before without incident). Thinking that perhaps the tub had reseated itself properly, I did a quick "Express Wash" with the few remaining items left in the tub & it went through the load with no problem at all.
I was thinking the problem was resolved & loaded a few of the sopping wet towels back into the machine to do a quick "Spin" - not wanting the excess weight from all the water to cause an error if I just started a regular wash load with them so wet. Well, before long, the washer stopped, displaying the Unbalanced error code (uL) - though without attempting to auto-correct for itself as it usually does... I thought at first it was just the weight of the water & the fact that the load hadn't been through a full wash to evenly distribute the items, and I manually shifted them. But the error code kept coming back, even when items were removed from the washer. So I turned the machine off and tried washing the few items on a "Heavy Duty" load, as I had read elsewhere this was sometimes best for heavy items. But this time, it went back and gave me the error code "F51" again.
I have read elsewhere on this site that error code "F51" can come from an item having been caught under the washing machine or having somehow slipped down on the outside of the tub. With the tub empty, I am able to fully rotate it without any problem, though - so I am thinking that must not be it.
Can it be that the tub is somehow not seated properly? Or do you still think that it is a loose connection?
Thank you again for your patient help!