I've been running a 2 low for about 8 years, have always thought it was one of my best modifications. It has never slipped out and I use it A LOT! I use 2 low like some guys use an ARB, leaving it in 2wd until I come to an obstacle and then slipping into 4wd. Since the NP231 is shift-on-the-fly, you don't have to stop to shift into 4wd. Running on hard surfaces in 4wd can stretch the t-case chain over time (I stretched one in a year), but I've never stretched one since getting 2 low. It also makes easy/moderate trails fun because you have to pick a 2wd line rather than not having to drive at all (with dual lockers). JB Conv used to make one, but they quit making it because they couldn't (or didn't want to) compete with Tera MFG, who makes one now.
If you get a 4 to 1, and a 32 spline SYE, and a 2wd low kit, and add larger chain and sprockets, you are approaching Atlas pricing. But, a 2 low and SYE is still much cheaper, and well worth the money. If anyone is having problems getting their t-case to shift properly, or pop out of gear, try replacing the small black plastic bushings in the shift linkage.