The Nite-Ize PR upgrade does work in any PR-base light, but the emitter is a little lower than the filament on an incandescent bulb. On most fixed-focus lights, you will be stuck in the flood setting, which isn't very impressive.
For a 2 cell work light I much prefer the SMJLED PR2 drop in bulb to the nite-ize mag LED replacement bulb. The SMJLED is brighter and whiter, with superb battery runtime. The SMJLED is a far superior performer without getting into higher current solutions. Do a search here in the forums for more input. You can buy these here in the "Sell" forum for around $6. I just received 6 more of these beauties from "TrueBlue". Super fast shipping!
I found the NiteIze MagLED bulb to be dim with a bluish tint. I use this bulb in my Dorcy 4 cell dome lanterns/flashlights. I used this on a recent camp outing and was wishing for a few more lumens. I am tempted to re-wire them to 3V so I can use the SMJLED PR2 instead.
Having had both the NiteIze and SMJLED PR bulbs, and trying them out in 3 different 2-D cell flashlights, I have to second everyone's recommendation here. I thnk you are likely to be disappointed with the NI.
Unless your 2D has a focusable beam, you are likely to get a large "donut-hole" style flood out of the NiteIze (it did on both of my non-focusable models). The light output is also a lot lower compared to the SMJLED. Oh, and the SMJLED is cheaper around here.
I've bought SMJLEDs from both Lambda and TrueBlue here, and have had great service from both (I believe Lambda is out right now, but check with TrueBlue).