My jeep has 100k miles and has run fine so far however, two days ago I got into it, started it, went to put it in reverse and nothing happened. I mean, the truck wouldn't go forward or back... the shifter feels like it is not connected to anything--just free front to back. I tried to move the 4WD lever and that didn't budge either. What is the problem?

If your "check engine" light is turn on, I suggest scanned the PCM to know exactly what fault code is stored there. Additionally, could be some problem with the "temp sending unit".

Keep us updated.

ecu is taking up more voltage as a back up reserved. Try removing the battery once it is loaded in the evening and put it back in the morning and try to start.Trial and observation will pin point the main cause.

The cables may just need to be cleaned real good or the ground wire from the battery to the fender wall may be loose or broken.

Could be the battery also the alternator AutoZone does this check for free other places charge about 45$ to do that check

Shift linkage bushings under the air filter box are probably gone.
http://www.boogerracing.com/boogerbushings.html

sticky valve in the trany... they have a conditioner for this problem./......you pour it in and in 2 days u should notice a difference make sure you get tranny conditioner.. nothig else

check your linkage from shifter to transmission.or if its a cable shift the cable may be broken.Smell the transmission fluid see if it smells burnt.if so then tranny reverse has went up

This could be something simple. Your starter draws far more power than anything except for the a/c clutch (not in play here) Since all accessories are functioning, it's likely that the battery terminals are oxidized. A high-draw item will actually break the connection temporarily. This condition will also change with humidity and temperature. Remove them and use a small wire brush or sandpaper to clean the battery posts and the inside of the connectors on the wires themselves. If you have done this and still have a problem, then you should start looking for other larger problems.
Good luck

My suggestion would be to replace the fuel filter, iver had this problem in my vehicles and replacing the fuel filter always seems to fix it