I just replaced the plugs and plug wires in a 1999 gmc jimmy and now have a distinct knock when I shift from park to reverse or drive. what can cause this??

You may have inadvertently disconnected the shift cable at the transmission, or the cable end may have broken.

check for a possible short; [open wires touching each other] when driving things move around, when standing still everythings stays still, could also be a porrly connected wiring harness

On the 1995 Jimmy, the shift cable can come loose on the mount on the side of the transmission where the cable meets the trans. When it slips, the alignment goes off and causes the driver to not be able to put the trans into park. If you take a long stick you can push the shift arm on the trans to the park position and your trans will go into park.....what you (and I) really need to know is how to take the shift cable mount apart and reset it.

knock sensor problems arise from ---wiring --poor connections --knock sensor----insecure KS--KS incorrectly tightened ---wiring short to positive --wiring short to ground ---ECM
as you can see , there are more causes than the KS itself so care should be taken in the installation, tightening , wires and make sure that the area is water proof

Sounds like the cat has fallen apart blocking up the outlet ,causing the vehicle to loose power

Bring up TDC on no. 1 cylinder, the intake should have the fire on top of it

possible internal problem, ck sun gear in the trans input shaft could be stripped out it would cause that problem.

Found this maybe it will help your problem:

To shift out of park one or two things must happen. First when you depress the brake a solenoid or cable releases a lock. A defective solenoid, relay or a blown fuse or a broken or improperly adjusted cable can be the cause.

Some console shifters have a bypass. It may look like a place to insert the key or a round plug that will pop out.

The button on console shifters when depressed pushes down on a rod that runs down the shaft. That rod releases a lever designed to keep you from accidentally knocking your shifter from drive to reverse. The button or rod can wear or break.

replace the cap and rotor with gm parts or you will still have an issue

Need to have the Transmission Linkage Adjusted underneath. You may have hit something that knocked it out.