66 TH400 transmission question

[Mossbeard]

Is this normal?

I have a TH400 behind a 327 in my '66 C20. When I shift from park to drive (or reverse) the whole vehicle lurches and there's a thump like I've been rear-ended.

If not normal, where should I start investigating?
I was under the truck recently (fuel leak) and the drive shaft had no slop in it.

Also, the last time I drove the truck I stayed below 45mph but I remember feeling like it wasn't shifting. I don't have a tach but the engine seemed to be revved high. At what speed (roughly) should I expect to feel a gear change?

Thanks