I have an '87 Samurai with a 5 speed manual transmission. My back up lights weren't working, and I figured the switch was the problem. Bought a new switch, but with the new one in there I can't get it in reverse. The threads aren't any longer than the one I took out, however it appears that the reason it wasn't working in the first place was because a chunk of the plunger is missing.

Any ideas, thoughts, solutions? Thanks.

I have heard that on some reverse switches that the little plunger thing is too long and it binds the shift rail and won't let you engage reverse.  Try loosening the switch up a turn or two and see if you can shift into reverse then....if you can, then the switch plunger is too long and you need to find a different switch.
I am guessing this is an auto parts store replacement switch and not a Suzuki/Geo OEM switch?
I have used replacement switches from O'Reilly Auto Parts and they work fine....but that has been a few years ago.