Truck Repair: 2002 Chevy S-10 Blazer Transmission, oil pressure guage, car sense

Question
   My girlfriend bought a 2002 S-10 Blazer from her parents last month. It's a 4x4 with a 4.3L and an automatic, 52,000 miles and in great shape. Her Dad took good care of it cosmetically and was good about oil changes, but I have my doubts about any other maintenance.
   Two nights ago after work she couldn't get it to go into reverse - no prior problems or warning. It went into drive and goes forward fine, but won't go into reverse. The fluid was low (barely touching dipstick) when I checked it the next morning (running of course). The fluid was a little dirty,(I've seen way worse) but doesn't smell burnt.
  At the same time, she also noticed that the oil pressure guage which she said usually is at or above 'halfway' (40 on the guage) was low and bouncing between 20 and 40. The tach was also bouncing but she didn't notice the engine revving or anything. When I checked the guages everything seemed normal. I can't recreate the guage problem but it still won't go into reverse.
   Could this be a fluid/filter problem or are we looking at a tranny rebuild? Also a thought since everything on this truck is electronic, could it be a sensor or some engine/tranny control module? Especially after she mentioned the guage fluctuations. I have good car sense but tranny's are definitely my weakness. So are cars that I can't see the engine under all the wiring.
   I will probably change the fluid/filter just to satisfy myself. But I wanted to get a second opinion on the situation to know we aren't getting taken for a ride if we take it to someone and they say we need a rebuild. We're new to the area and don't know any mechanics reputations.
   I don't think this would affect anything, but the weather was snowy and dropped to below zero that night and so far since then.
   Thanks for any advice, even though I fear the worst.

Answer
Hi Don,
Don't give up yet. Get it somewhere warm, and see if there might be some woter frozen somewhere, like in the electric shift mechanism.
Maybe hang a droplight near it.
Heck...might even spray it with de-icer.
I haven't seen the problem, but I'm in Oklahoma.
I also don't work on transmissions much, but that's an educated guess.

Van