Another vote for the NV3550. I love mine. Great for around town. I never got to really wheel it once I got it installed...but I did drive it for about 7 months before I disassembled the trcuk for the frame off. Its a very clean install. The AA parts fit great and are very durable.
If you are looking to do a bit more offroad and hard-core wheeling, then I would go with the NV4500. The NV4500 gear ratios are almost the same as the ZF ~5.5:1 1st and ~.72:1 OD, the rest of the gearing [2nd, 3rd] (which agian is almost the same as the ZF is about the same as the stock three speed--which are all OK, but not as ideal for street driving, cruising and mild wheeling as the NV3550--1st, 2nd, 3rd are awesome in this tranny). However, the NV4500, like the ZF is a heavy duty tranny-- very beefy, very strong, but vs the ZF its relatively short. Your driveline will only increase a few inches vs the ZF which is a huge assembly. If your running alot of susp. lift, then you may not need a new rear drive shaft, but will probably need a front. I would imagine from my experience with their 3550 kit that the AA-4500 kit is also a great in fit and function. There are also a few individuals that make the bellhousing and D20 adapters for these kits...their price is cheaper than AA, but that adds a bit more work for you factor in sourcing the crossmember, bolts and other small parts (which can be more $$ byuying separate)....vs the AA kit.