Hi- I came upon this site because of the same problem! This is what I found-

"My 850 got stuck in park today, so husband told me to use shift lock button to get it out, it did come out but now my tracs off light and abs light is on. any clues? thanks kelly"

"The ABS/Tracks problem is not normally related to the shifter in itself BUT it's also possible that your cars electrical part of the ignition switch is failing and that could cause both problems together. If you move the key in the ignition switch (NOT engaging the starter with the engine running), does the ABS light go off or will the shift lock work by moving the key, then depressing the brake pedal? "

"yes, checked out volvospeed last night so this a.m. i tried the ignition shake with out starting and the lights went out. kelly "

"Easy fix www.volvospeed.com look under micro switch replacement..
something your husband.. can do in 1 hours.. save some big $$$$$$$$
some one even said buy the part at Radioshack for nothing... and it work the same... compair to dealer price....."

"Sounds like you need a shift lock solenoid microswitch, was a common problem for many years and it's not overly costly if you choose to have a dealer do it for you."

"Good. Volvospeed shows how to replace the switch. Have fun. You can get the parts at a good discount from Dan at volvoparts.com (or any number of other places). email or phone him."

"You need to pull the A3 codes and see what they say. Volvopeed.com, Ozbrick, a Motors or Haynes manual will show you how."

"Sounds like it may be the brake switch on the brake pedal or the electrical portion of the ignition switch. You may be able to wiggle the key in the ignition switch to see if that helps. If you push on the brake pedal and wiggle the ignition switch or slightly turn it (engine running) you may hear the Park interlock relay clicking on the passenger side and you may see the ABS light go off and on. Don't turn the key so far that you engage the starter.

Volvo has problems with the abs control units on this car and 850s. I would take it in to the dealer to have it tested."