High beams are stuck on..

Bright lights wont shut off ? Assume you mean high beams ? Reading how they work might thing high beam relay has stuck contacts inside .

Headlamps
The headlamps may be turned ON 2 different ways. First, when the driver places the headlamp switch in the HEADLAMP position, for normal operation. Second, with the headlamp switch placed in the AUTO position, for automatic lamp control (ALC). During ALC, the headlamps will be OFF in daylight conditions, or low beam operation in low light conditions. Battery positive voltage is supplied to the low beam relay and the high beam relay in the underhood fuse block. When low beam headlamps are requested, the body control module (BCM) supplies ground to the coil side of the low beam relay. This activates the relay and the relay supplies voltage to the left hand low beam headlamp through the LH LOW BEAM fuse and to the right hand low beam headlamp through the RH LOW BEAM fuse. Ground for the left hand low beam headlamp is provided at G101. Ground for the right hand low beam headlamp is provided at G102. When high beam headlamps are requested, the body control module (BCM) supplies ground to the coil side of the high beam relay. This activates the relay and the relay supplies voltage to the left hand high beam headlamp through the LH HIGH BEAM fuse and to the right hand high beam headlamp through the RH HIGH BEAM fuse. Ground for the left hand high beam headlamp is provided at G101. Ground for the right hand high beam headlamp is provided at G102.

Faulty switch.

What year is your GMC Jimmy? This will help me give you the information you need.

Someone would need to check for voltage output on the dimmer switch for low and high beam.

There is a headlight relay in the engine compartment fuse box, but its after the headlight switch and before the dimmer switch, so it powers both high and low beam circuits.
I would check the dimmer switch.

It's a shorted or worn out headlight dimmer switch mounted on the column.

sounds like the high beam switch is bad, it controls the power for low and high beams

well as a tech i dont recomend it but all you got to do is swap the wires on the headlamps at the bulb you will see 3 wires one is a ground the other feeds low beam the other high beam . take a test light and find the one thats hot and cut it and connect it to the other blade , leave only one wire on the low beam terminal so you dont short it out .

you need to change the dimmer switch it has gone out

pull on the signal lever toward you (ceter position). the signal lever controls the head lights either high beam or low beam. if you pull on the signal lever toward you again, you will have high beam temporarily. and if you push the signal lever towards the front, you will have the high beam locked.