Jack up the truck with jackstands, start it up and select 4wd. all 4 wheels should be turning

 

My TPS (trottle position sensor) was going out and my engine would get loud at idle. Another option.

If your i 4x4 mode the front driveshaft wont turn if your in park. If your in park and the front dirveshaft turns by hand, then your not in 4x4 mode.

There ahve been a couple of people who had problems with the puter thinking the t-case was in 4lo. The puter adjusted engine performance to as if the t-case was in 4lo, however the main clue there is your top speed is like 25mph. The fix was in hte wiring harness.

Make sure your t-case and tranny wiring harnesses aren't full of water/moisture, and they are tightly connected.

 

"Make sure your t-case and tranny wiring harnesses aren't full of water/moisture, and they are tightly connected."
How do I check that?

The loud noise is only there on first and second gear, then does away as I gain speed. No noise on idle, except when I press gas when in park.

BTW: Thanks everyone for the advice! You've been extremely helpful!

 

Yah I would think you would hear the fan if the clutch was bad at idle. Mine is also gone its stuck on at all rpms and temps. The clutch should be disengaged at 2500rpm and should be engaged on at like 150-200 degrees.

 

By taking them off and looking for water in the connections. Use WD-40 (Water Displacing 40th attpempt) on the contacts. Then cover the contact with di-electric grease.