speedo question

[Irish1941]

Easy test:
put a drill on the cable underneath at the t-case and spin it up.
Have a buddy watch the speedo inside.
No bounce: gear swap is the problem
Still bouncing: cable is problem.
The gear could have only come from a '89 or down Blazer. All electronic in 90-91.
As was said, as long as there is the square cable end coming out of the sheath then it should work.
Also, remember most of the late Blazers (87-89) came with a 2.73 or 3.07 gears so the t-case gear you put in is more than likely wrong and giving you wrong speedo readings. Not sure what gears you have but pretty sure it's reading wrong.