Why is 1991 ck1500 dash brake light stuck on speedometer won't work too any ideas?

If the speed sensor has no input,the ABS light will come on.Check your speed sensor first. The 4X4 sometimes has 2 sensors one on the transfer case and one on the tail shaft. on my truck the tail shaft sensor was not used , so when the transfer case sensor failed,I hooked up the tail shaft sensor and everything went back to normal.