[Balanor]

According to what RedKnight found, it sounds as though Rangers can choose whatever deity they like, however they are typically nature-based.

http://thaypw.7th-sphere.net/showthread.php?tid=403

According to the 3.0 Forgotten Realms Campaign Setting pg. 24

"Unlike clerics, druids, and paladins, Faerunian rangers do not have to choose a patron deity until they reach 4th level and acquire divine spellcasting ability (without a patron deity, a ranger cannot cast spells) Many rangers choose a patron deity before then, but others start by devotion to a ranger's way of life instead of to one of the gods. Rangers and druids have similar preferences for deities, although some rangers with odd interests (such as hunting undead) choose different patrons."

Though not exactly specific, it seems to insinuate that not 'Only' nature gods are needed to be worshipped for rangers to obtain divine spells, it's just the norm. Makes sense to me considering some rangers do devote themselves to non-nature based life quests and what not.

Still looking through, but for now this was under the class descriptions for faerunian specific info in the campaign sourcebook, 3.0 edition.

I'd say rangers of certain deities would be good candidates for DM harrassement though unless they have a good backstory. I'd have a very tough time believing there are any rangers of Sharess or Lliira for example - not impossible, but the PC in question should have a pretty good reason as to why they picked that god.