Hey,
As a currently 38 cleric who has soloed the vast majority of my levels, I can give you my perspective on cleric soloing.
First off, an item that will be EXTREMELY useful in soloing up until roughly 30 in the Bonethunder Staff. You can get it from a quest in qeynos at around level 8:
http://everquest.allakhazam.com/db/quest.html?quest=573
I leveled to around Blackburrow until 13 or so, then I did bandits in WK, you make a TON of plat & faction from bandit sashes. If you wanted to solo undead for those levels, Befallen is an excellent place to go. The drops are great and the exp bonus is very nice, plus it's about 80% undead. You can solo there into the mid 20s if you want to.
Unrest is essential for clerics. I was soloing yellows there as a completely untwinked erudite cleric until my upper 20s. You can theoretically solo there into your 40s if you're really hard core about it. The bloodstained tunic drops here, which is a really good +4 wis chest piece. The bonethunder staff makes soloing here extremely easy and minimizes downtime.
I've actually found that clerics are very good at soloing live mobs as well. Undead are definitely better, but I've been soloing the king spawn in Permafrost from 35 to 38 and the exp is ridiculous. The mountains of Rathe might be good for upper 20s-low 30s, there seems to be a lot of undead roaming around there.
The undead side of LGuk is good if you really know what you're doing. But, never underestimate a cleric's ability to solo live mobs. You can split any camp and solo blues all you want. As far as undead go, Kithikor is an excellent place to go. You can kill the undead cleric at around 30 or so and get a +6 wis mace.
the actionable takeaway of this post: get the Bonethunder staff & go to Unrest.
Have a good one,
Alkaiser Baneofmistmoore 38 cleric of Prexus