1994 Ford Explorer Slippage in cold weather.

I live in Wisconsin and it's winter season right now. The problem started when the cold weather set in.
When I park the Explorer for more than 6 hours, then go to start it, put the shift into drive it seems like first gear won't engage & the transmission stays in neutral for about 10 seconds. None of the gears work at this time. The only thing I can do is shift back into PARK and then try to shift into DRIVE again. Sometimes it will engage into first and other times I have to redo the whole process. The other situation is when I'm approaching a stop sign/red light I can hear the transmission decellerating, but there's no grinding. Is there a simple fix or is there an additive that I can use? How long before the transmission will go out completely if this problem persists? Please send your comments or recommendations. Thank you. ~Ford94wis