I'm new to the hobby.  I've recently got a 65 Pontiac Tempest with a Turbo 350 and a standard rear-end (non-posi), with a B&N ratchet shifter on the floor.  Once in a rare while, I'm not able to get the car out of park, even though the gear selector lets me move throughout all the gears.  Even worse, there have been a few times when the car was rolling very very slowly and locked up.  Its a strange feeling - it could be stuck in park, but it seems to feel more like the gears in the rear end are locked up.  Yes, I've stopped driving it.  I'm trying to isolate the problem to a single major area.  I don't want to end up replacing everything and blowing my budget.

Thanks in advance.  

Have you pulled the diff cover and checked for metal shavings in the gears? You would definately have to remove the diff cover and the ring and pinion gear to check, as well as replace the diff fluid completely. this would definately be the cheap way to go if there is no severe damage to the gears and no metal shavings in the fluid you may eliminate differential lockup and focus on the transmission. Good luck  

Yep, first best thing for you to do is pull the rear end cover and look for debris or signs of a ring gear problem.

Take the driveshaft out so you can turn the rear without the trans being in neutral.  

I took off the cover, no obvious signs of shrapnel.  The fluid was dark, and the gears appears to spin freely.  Replaced the fluid, went for a short drive.  It didn't lock up, but I heard a few very loud clunks.  Wierd.  Do you think its really 'falling into park' ?  

Ok, so I found out that I have a TH400.  If I were simply going to replace the rearend gears, how do I pick a good match?  Should I just get a set of 3.08 or 3.23 for now?  What specs do I need to know about the rear-end in order to safely buy/order a gearset?  Is there a code stamped on it?  Do I measure it?  Count the bolts on the cover?  Sorry - learning as fast as I can.
Thanks in advance.
T  

This is happening to me as well.  I've been troubleshooting the problem online and came across this thread (I actually just got a 67 Camaro) and thought I'd post a reply.  I got the car a week ago and it is also not a posi.  The rear end randomly and intermittently locks up.  It happened in idle and then also at about 20 mph and came to a complete, jolting stop.  It almost felt like I had been rearended or like I ran into the back of someone.  All the times have been on a straight road.  I'm thinking immediately that it's either the spider, pinion, or ring gears.  I haven't checked the rear end yet for trash.  I've actually been out of town.

Any other ideas??
Thank you!