My 2007 Trailblazer is stuck in 4 wheel drive!! Help.

could be the relay or the switch solenoid on the transfer case

May be broken 4WD actuator (quite common) which has stuck between engaged and disengaged. Remove and inspect actuator, but first examine all linkage for damage or having come loose.

Switch may be bad. Change switch should fix it.

Have you had codes checked ? While connected to a good scanner that will check transfer case switch to show if switch is working, and it could be the switch on the side of the transmission.

More than likely it is the switch alot of electric 4-wheel drive switches have been known to go bad from lack of use

First of all, the noise is not coming from the rear. The noise is only happening in 4WD mode, that involves the transfer case and front differential.
When was the last time the transfer case fluid change, it is recommended that it be changed every 50K miles. It is possible that you have transfer case issues. It is not recommended to operate the drive system in Auto mode constantly, this will wear out the internal clutches.
If the road surface does not warrant 4 wheel drive, keep the selector in 2WD.

usually the actuator will still make the light still come on if its bad I would check the actuator

I had same problem you need to change the switch on dash and/or the actuator in the transfer case

pull atc 25 amp fuse in underhood fuse center and 10 amp 4wd fuse inside cars fuse box,your problem is in control box behind kick panel on pass. side ,corrosion build up,

replace the switch on dash that switches it from 2 wheel drive 2 4 wheel drive