Every time i put my truck in 4 wheel drive and i exceed my rpms and then i let off then proceed to down shift i hear a bang coming from directly under the drivers seat and i was wondering if anyone reported this cause i had my transfer case and front drive shaft looked at and my shop or ford dealer couldn't locate the problem?

this is because its not fully shifting back into 2 Hi. make sure vehicle is in neutral and idling, push the 4 Low button listen for a shifting sound or whirring noise. then push 2 Hi button. it flashes while it is attempting to shift you should hear a shifting clunk or whirring noise, then the light will stay lit. after shifting from 2 wheel drive to 4 wheel drive shift into reverse before selecting drive on the column. the rpm going so high is because your vehicle is still in 4 wheel drive. the low gearing results in high rpm. if it does not shift into 2 wheel drive : there is a shift actuator on the transfer case, check the cable connected to it. this cable is connected to the vacuum control pump deal that controls you cruise control. this pump is located directly under the battery box. the plate the battery sits on must be removed in order to see it. check any wiring there and check for vacuum leaks in the lines connected to it . this pump actually pulls on the cable going to the shift actuator. check that the cable is attached and the baffle on the pump is moving. you can manually pull the cable by hand, while having an assistant operate the buttons inside the cab. this will release the actuator in the transfer case allowing the transfer case to shift properly. basically its a locking mechanism, locking the transfer case in the selected gear range. it must actuate in order to complete the shift

if you are driving in 4H on hard surfaces and just flip the switch to 2H still on the hard surfaces it will react exactly as you describe
when in 4H as there is no internal diff action in the transfer case , a condition known as torque wind up occurs and places heavy loads on the drive line components thereby locking the gear teeth and that will not allow the separation on selecting 2W drive
if at some point one wheel looses traction for a split section, that torque wind up is reduced and the pressure on the actuator slams the selector out of engagement
4H should only be used in slippery conditions , mud, dirt ,snow,ice, grass etc where the wheels loose traction and allow that wind up to dissipate
if you continue to drive on hard surfaces in 4 H then transmission failure will occur earlier than can be expected

does it bang when shifting from park to drive ! if so its idling to high. should be 650-750 rpms. good-day !

it is most likely an automatic awd, 4 hi is full time 4 wheel drive, 4 lo is full time 4 wheel drive but at a very low gear ratio and should not exceed 25 mph or could cause damage. and DO NOT shift into 4 lo while vehicle is in motion. Auto is a 2 wheel drive but automatically engages into 4 wheel drive under heavy acceleration.

I also have a 2002 Envoy. Shifting to and from 4wh and 4wa can be done on the fly but shifting to 4wL has to be done stoped and while in neutral. Try that. Otherwise I'm afraid its a new computer for you. Its located under the left rear seat and you need a loan for one.

that is not normal
there is a problem in the drive shaft by sliding so fa into the diff the cv joint"U" joint is no longer directly in line with the kingpin and is binding up as the steering direction is varied
the center line of the joint has to remain directly inline with the turning point ( king pin, strut ) so that as the axle is turning and the steering is turned the joint is in the same plane
check why the axle is allowed to move in and out and you will have your problem solved

YOU WERE RIGHT. it does not have to be put in neutral. Just stop or slow down the rpms. Before switching to 4 wheel drive.

look on fire wall pass. side you will see 2 parts with vacume lines attached 99 percent of the time that is what went i had the same problem

first off you can not de driving down the road and shift into 4 wheel dr. ...... yah have to STOP put in park ,shift into 4 hi then drive do this and let me know how it works for ya........
inastuctions are or should be on the back of drivers side sun visor.......
rank my solution ty

HI! I have the same truck and to get it into four wheel drive shift to neutral first, apply the foot brake, move selector knob to fourwheel drive and then try to proceed. I hope this helps, John