Have a 98 GMC Sierra 4x4 5.7L Vortec 1500 and It will shift in 1st and 2nd but have nothing after that. I still have reverse an part as well. I hooked a reader up to the computer and it gave a code for a sensor. If i replace the sensor will it correct my problem how hard is it to replace and can it be done myself?

SOURCE: replace rear axel bearing on 4x4 5.3 Litre gas engine

Loosen differential cover and let oil drain. Remove cover, remove U lock from axel. Pull axel out. Remove bearing seal then bearing. Install new bearing, seal and reinstall axel. Reinstall U lock and new cover gasket. Fill with 75-90wt gear oil. Now I know that sound pretty straight forward. But there are some things to remember. The U lock can be difficult to remove sometimes. The most important thing is the seal, you will need to make sure it gets seated correctly and not torn when installing axel.

SOURCE: 2004 GMC sierra 1500 4x4 brakes

Have you checked the hardware on the disc brakes all the way around, as well as checking the rear pads? Wore out hardware could cause you to require more pressure to get the pads to move, as well as low brakes on the rear. Your braking system sends more pressure to the front of the vehicle because it requires more pressure up there. Less pressure on the rear can cause this to happen. I'm willing to bet the problem from the pedal is coming from the rear brakes somewhere and not the front. The last possibility that I can think if is a problem in the ABS solonoids, they might not be opening up all the way causing you to require more pressure.

SOURCE: 1990 GMC Sierra 4x4 1500

the connections near the front axle that activate the motor to engage the system aren't melted? That's a comon problem for that body style. If there good, sounds like it could be that motor itself. To figure out if it's you motor or transfercase, jack up the rear end, have a friend inside shift into 4 wheel drive, or try to, and put it in gear and apply some gas. If the front driveshaft is turning then it's something with the front end. If not, farther back in the system.

SOURCE: transmission wont shift into any forward gears or

4WD! the transfer case is not stuck in neutral is it? Fluid levels are good? Have you noticed a decrease in mileage or power prior to this problem? If no then time to take it in for service it sounds like. Transmission fluid is not consummed, so if it leaked out then you may be able to fill it up and get the leak fixed, if too late then its a rebuild on the tranny.

SOURCE: i have a 94 gmc sierra with 350 that has problem shifting to 2nd

Your problem likely resides with the hydraulic valve that operates the band for second gear, or lack of hydraulic pressure in the transmission. If the actuator in the transmission that operates second gear is getting sticky, then it will cause this. A filter change and flush may help, but it's likely time to start looking for someone to tear down your transmission, or replace it.