The GMC Jimmy Sierra was a SUV built from 1969 to 2005. It was based on GM's medium truck platform, the Silverado/Sierra. It came equipped with four-wheel drive that allowed the driver to switch between rear-wheel drive and all-wheel drive. This allowed the SUV to drive equally well on the road and off the road. However, this entailed the use of complex transfer cases (essentially a second transmission), several transaxles and two shifters. Over time and with hard use a number of issues can crop up from warped axles to broken gears.
Look beneath the center of the GMC Jimmy Sierra, preferably under the SUV on your back. The transmission will be located right under the shifter and attached immediately to the rear of the engine. The transfer case will be near the center of the SUV and is connected by axles from the transmission and to the front and rear differentials. Look for any signs of damage or leaking fluid. If either component is cracked, then the Jimmy is not safe to drive and needs to be flat-bedded. If the components are leaking then they just need new gaskets.
Start the GMC Jimmy and let it idle at first. Shift the transfer case from 2H (meaning 2 rear wheels high gearing, ) to 4H ( meaning 4 wheels in high gearing). Feel for any resistance from the shifter in your hand, it should be slight. If the gear will not move at all, then the transfer case may be seized up and needs to be replaced. If the gear moves in place but will not engage, then the teeth could have been knocked off the gear, necessitating a rebuild. Shift into 4L ( meaning 4 wheels with low gearing) and again feel for undue resistance. Also, listen for any grinding that would indicate damage.
Shift back to 2H and then shift the transmission into drive or first gear, if it is a manual. Again, listen for grinding and fell for any resistance that would indicate that the gears were not meshing properly. If it is a manual then pay attention to the resistance of the clutch, it should not be too hard or too soft.
Drive the SUV forward in 2H and pay attention to the power. If you are having to rev the engine high into the RPM redband just to nudge forward, then the transmission is not transmitting torque efficiently. However, this could be a problem with the rear differential as well. Shift into 4H and watch the RPMs again. If you are still going into the redband just to go a few miles per hour, then the transmission is at fault or both differentials are starting to seize. If the Jimmy moves better in 4H then only the rear differential is at fault, not the transmission.
