1999 Ford F-250 super duty

Engine Size : 5.4
Question: The speedometer bounces around and sometimes the transmission shift to neutral. Went to the dealer, they changed the sensor on the differential and the ring inside it, checked all the wiring relating to it, yet it still bounce around.

Answer: Normally the speed sensor (VSS) in the rear end fixes this problem. If not, then the gear in the rear end housing is the other usually cause for this problem of the speedometer bouncing. Very surprised this did not fix the truck. Still sounds like a wiring problem. Suggest taking back to the dealer, since you paid for a repair that did not fix the problem. They should spent more time on it for you without charging you any more diagnostic fees.

4 Wheel Drive Not Engaged

Ford F-150 XTR 4Wd. Engine Size : 5.4.
Question: Indicator light shows 4x4 engaged but front wheels are not.

Answer: There are many, many reasons that the front wheels will not lock. With this limited information, it would be impossible to guess. All you are going to read on those forums is everyone telling what is wrong, because they had this same problem one. It could be electrical in the switch or control module (if it has a module- you did not tell me the model year), could be a wiring problem, could be a problem in the transfer case, front differential, etc.