[Wally Arntzen]

On my 88 I had a similar problem in Arizona a couple of years ago. I don't know if the If 99's have air oporated shifting on the transmissions or electronic.
My is air operated and there is an air shift pistion on the passenger side of the transmission that moves forward and backward depending what shift number you put it in and that will move as you change it at your shift handle or switch.
That piston got dry on mine and would not move. I was informed by my friend who is a bus mechanic wizard to get under and squirt it with WD40 or another loosening lubricant and it released it immeadiatly.
If you have the air operated switch make sure that you have the coach brake on when you go under and do this. Better yet turn off the engine and do it because you can move the piston back and forth without it running.