I believe the transmission linkage on my 2001 tahoe has somehow become disconnected. I'm looking for a diagram of this linkage. It is in park and when I go to change gears, the shifter just about falls...has no resistance at all and it never leaves park... Can i manually reconnect?

Block all wheels front and back so you don't get squished.
Have a close look at the shift linkage with the key on but the engine off with an assistant slowly shifting through all gears.
If you can see the shift arm on the side of the transmission going through all positions and can hear it clack into each gear, then the issue may quickly become a need for transmission rebuild.

If you're talking about the Park Neutral Safety Switch, it's on the linkage right up against the transmission, driver side under the hood. If you're not talking about the Neutal Safety Switch, then spell out exactly what you want and do not abbreviate.

First thing is to get safety glasses,block wheels securely !!...set emergency brake..,get a good flashlight.Disconnect negative battery cable...Some can wear inside the steering column,some can be a worn linkage cable ...The lever on the transmission located under vehicle Trans at lower part of trans,driver side can become loose or possibly damaged...First get under vehicle with safety glasses and observe operation of the lever where shift cable is mounted while assistant puts it in park...make sure the trans shift lever mounting nut is tight before shifting..If assistant can't get it in park use your hand and push forward on the linkage arm and it should go into park manually...If this is the problem you need to change shift cable...Some wear inside the column where steering wheel is mounted..If cab shifter feels very loose ..You will have to remove steering wheel and column cover to inspect."..The cable runs from trans to the steering column..Look between gas and brake pedal and verify cable coming up through floor board and runs to steering column...Double check shifter lever at trans is tight because it is mounted on a splined stud and it can strip the stud threads if it becomes loose..If loose remove lever and inspect splines for damage..Call dealer and make sure they have the part..Remove old cable from trans and put it in park with your hand...Remove and replace cable..Be sure lever inside cab and lever on the trans are both fully engaged into park while cable is removed before installing new cable....Attach new cable.....

Sounds as if your linkage may have become disconnect from the transmission.

First you should make sure the problem isn't in the transmission buy disconnecting the shifter cable at the transmission and push the lever all the way into park. Now push the vehicle hopefully the vehicle won't move indicating it is just the cable. Next take a look at the bracket that holds the shifter cable make sure it's not bent and properly installed. If ok try adjusting the cable which is right where you disconnected the cable at transmission.

check linkage down at transmission and nut on bar at transmission where linkage connects

Disconnect the shift linkage at the transmission and see if you can move the transmission lever into park manually. If you can't, then the internal trans linkage has a problem and pan will need to come down to find out why. If you can do this, then re-connect and adjust the linkage.Check any bushings on linkage pivot cranks...If a bushing is gone it may not be allowing linkage to move all the way to where it needs to go.

A BAD BRAKE LIGHT SWITCH OR FUSE.THE BRAKE / TRANSMISSION SHIFT INTERLOCK SOLENOID IS BAD OR BLOWN FUSE.

the linkage is probably binding.disconnect the linkage at the transmission and see if it moves properly.