The distinctions are a bit vague. Here’s my view of it, as I have come to understand.
Essentially, Loa are any being of higher power than the trolls themselves- in most cases, they are wild gods, similar to the Night Elven ancients, and have similar powers as such. However, this is not the case with all Loa, as many other beings have been referred to as the Loa or ‘Gods,’ such as the Old God G’huun.
Given this variation, it seems as though many of the trolls may indeed worship different beings to create these different classes. In some cases, it’s actually confirmed, though in others it’s a bit less obvious.
Firstly, for both Zandalari and Darkspear Druids, the primary Loa they follow the teachings of is Gonk, the master of shapes, who taught trolls Druidism. Officially, Gonk supposedly taught trolls how to serve “all nature spirits at the same time” rather than just a particular Loa. As such, this confirms that Troll Druids take a wider view in which Loa they honor, and that most trolls honor a single Loa above the others.
To follow this, it seems to imply that each Shaman or Priest serves only a single Loa, or at least have one that they serve above others, which they draw power from. Functionally, these two classes seem to be interchangeable in troll society- troll priests can often be seen, for example, using shaman-like totems. As such, the separation would be in powers a given Loa can grant to an indivdual troll (for example, priests of Rezan can utilize Light based powers, or at least powers that are functionally similar to Light powers seen in other races). However, which Loa a given player character communes with is left vague for these classes.
In terms of Paladins, they are confirmed to have worshipped Rezan (when he was alive), and gained light based powers by doing so (or, again, functionally similar to light powers of other races). The fact that some of them kept their powers after his death seems to imply that Rezan was not necessarily the source of their powers, but was able to connect them with the Light itself, using their faith as a catalyst for wielding that power, given how conviction and will are more important in wielding the Light than an actual specific code or target of worship.
That’s a bit of lore, thrown in with a couple of my own interpretations. Anyone feel free to critique if there is canon lore that I missed.