With a Paladin you're looking at a gap between about 44 and about 53.

Nothing is very good with a paladin until you can take those Shamans into Stratholme. The reason I say about 53 is that unless your paladin is insanely geared you're not going to grind XP faster than you can quest it. At 53 the Shamans can actually start to sort of help.

ZF is terrible for a paladin for the reasons already mentioned. BRD is the same situation. It simply isn't going to be worth your while over quest grinding without a higher level helper. Uldaman is actually slower than SM.

If you want the XP gravy train to continue in that gap there is only one answer. A warlock. I subbed a warlock in for a paladin at 45 when I got desperate and ran out of ideas for the Paladin. I went to ZF, put the shamans on AF and ran through the instance insta dotting everything. I was doing about 100k per hour. That will easily take you to 52-53 in a few hours. From there you need to simply bang out a couple more levels and then go to hellfire ramparts. You will do a solid 60-70k an hour in Ramparts with a paladin tanking it for you and it will only speed up as you level and less stuff is red. The bonus is you start to collect gear and working on your instancing strategies ;p