Today I needed 4 wheel drive I shifted it into 4 low and you can hear and feel the clank but the front tires don't even grab. Feels like its still in 2wd. Just sits and spins

 

Probably one of three things, differential vacuum actuator, front differential shifter fork or vacuum solinoids...all three are problematic..

The transfer case shifter motor is as well but if you hear it engage, it is most likely fine.

I replaced my shifter fork, actuator and shifter motor in the last 13 months..

 

Is their a way to tell what it could be or is it just hit or miss on those three things

 

check to make sure that acuator is getting vacuum. Also take some wd-40 and lube the shift fork and make sure you can move it. I have replaced everything at least once.

 

It's all in the diagnostics..If the front actuator is moving to it's full extent, it isn't the vacuum or the actuator, it might be the shifter fork in the front diff. There was an upgrade for the shifter forks as they came from the factory plastic and fall apart eventually.

Not meaning to be rude but the search feature is a great tool, I could go into and explain it all again, but there are smarter people than me that have already done this..

Mine did the same thing and it was the actuator (easy to replace), this year the transfer shifter motor (easy to replace) went and shortly after I pulled the diff (not so easy) and there was a plastic slider on the shifter fork missing..