Hello, this is my first post and I was hoping to get some advice. My dad and I recently bought a used Bronco II. It ran great for a week, but while driving it a few days ago the transmission suddenly stopped engaging. It doesn't seem to work in any gear but reverse and you gave to rev the engine to nearly 4000 rpm to get any movement at all. This morning it worked great for about 10 minutes in all gears, but then stopped engaging again. It has stayed at or below freezing the past few days, if that is relevant.

So far we have checked the fluid and vacuum modulator line. We don't know how to proceed though. Any advice at all is very much appreciated.

 

The fluid in the torque converter is probably a little low, and the cold has caused it to shrink a bit. Check out your transmission pickup filter, make sure it's able to do its job. That can cause a low fluid level in the TC, and cause exactly what you are describing when it's real cold outside. If that's not the case, could be a valve body issue.
One thing you can do is check the "warm" level of the fluid, and then check it stone cold. If it's high for the cold measurement, but normal for the warm measurement then the fluid from your TC is making it's way back into the transmission.