This got me stumped and frustrated. Started out with the service 4wd message. I replaced the selector switch but for the first time it didn't fix the problem. Next I checked the frame ground wires under driver door. Finally I took to the dealership. They charged me $99 to diagnose the issue. They had it in the bay all day which makes me wonder if they are responsible the next symptom. When they finally brought my truck around they said they finally got the 4wd selector switch button lights to come back on after testing the switches voltages. (I never had an problem with those lights). They said the switch was good and everything points to the tccm. The switch lights were off and on for 2 days until they started staying on again but then tach started bouncing under load. Up and down about 1000 rpms distance. After all this I replaced the tccm with a re-manufactured preprogrammed unit. That did not fixed anything. Then last night i noticed that buckling my seat belt / stopping seat belt light blinking in cluster, stops the bouncing tach. Until i press the gas.
In short, I have service 4wd and bouncing tach. Tried replacing 4wd switch, checking ground wires, replaced tccm. Bouncing tach did start until I took to dealership. Any ideals what to try next?