PNRD switch or position/range switch of transmission

2003 Chevy k-2500 HD Duramax, allison trans. 90K miles.

Transmission problem. It will sometimes not indicate the gear it is in on the dash. Other times it will. Sometimes it will not go into reverse (says something like "inhibited" on the dash). If it does go into reverse it might slam hard, as if I just ran over something. Usually it does not give me any problems until I have drove for a little bit.
Update. Today it totally failed. I had to drive home in 3rd gear. Wont show D or R now. The dealer said I need to replace the RANGE SWITCH/POSITION SWITCH on the side of the transmission. They ordered one for me today. Anybody have any tips on how to do this myself? One guy told me to put it in Neutral when I do it, and a clip will self adjust it. Any help or tips would be great, my repair manual does not cover the Alison or Duramax part of my truck. Thanks, Mike