2002 CHEVY ZR2 S 10 4WD SOMETIMES IT WILL ENGAGE AND SOMETIMES IT WON'T. WHAT WILL CAUSE THIS PROBLEM?

Vacuum hoses and connection under vehicle on transfer case. Sometimes vacuum switch goes bad also. Worst case, the control module for the 4wd

A six cylinder Trailblazer does not use a timing belt. It has a timing chain.
Are you sure you did not cause a problem with the cam or crank sensor ?

There is a low pressure switch on or near the drier that can be jumped, it is also a common problem with them.

Same problem here, found break in wiring harness at cross-member behind cab. did have bed removed for easy access. Ground wire guilty party.

Sometimes the anti theft security can cause the starter to be turned off. It's a pretty common thing on GM vehicles. Read about it in this link. Compare symptoms.

http://vatspasslockpasskeysecurityhelp.yolasite.com/

Do you see a light on the dash that says security when this happens? http://vatspasslockpasskeysecurityhelp.yolasite.com/

a/c compressor froze up? Won/t turn?

on those model of trucks you have to back up 15 to 20 feet sometimes to properly engage or disengage the four wheel drive if that dose not work the try manually shifting the transfer case on the upper passenger side sometimes the linkages slip or just stretch from years of use,and can usually be adjusted.Also make sure your mounts are all in good shape as well as the engine mounts,bad mounts cause binding and will also cause you the same kind of problems.

there is a vacuum actuator under the battery tray. it is the most common cause for this issue. also the cable that goes from that actuator to the differential stretches sometimes causing it to engage and not engage depending on how bad it is.

if you have any more question please ask/

if it turns over there are a cocuple of things to check. crankshaft sensor and the coil pack maybe the computer.id start with the crankshaft sensor