You can test a relay with test leads and a voltmeter.

 

Or swap it with a different known-good relay from another slot.

 

Thanks Extinct, I'll "relay" to my mechanic.....

 

Question: Have you been either:

1) Been messing around with a Nanocom or coding device by chance?
2) Added a 2004 center diff lock to the truck ?
Because -- if you screw up the settings for either the the transfer case in Nanocom, you can get exactly what you described. I know because I did it, had a rapid clicking and couldn't shift into gear.

A few more ideas to pinpoint... these are all things that will cause that rapid clicking from under the dash / steering column area:

1) brake light switch (sounds like you changed that). PS - Also, and it sounds insane but makes sure all of your brake light bulbs are good and working. There is a scenario by which somehow they backfeed current thru the brake light switch and cause all sorts of hell if a filament breaks in just the right way on early D2's.

2) shift interlock (under the shifter there is a switch that prevents you from switching out of P unless brake pedal is depressed)

3) fuse box is corroded from water ingress (fuse box under drivers steering wheel)

4) As above, transfer case interlock settings are set incorrectly from factory settings (ie someone has changed interlock for either gear shifter or transfer case recently in Nanocom, or truck is new to you)

5) XYZ switch under the car is bad - can also cause haywire stuff like this