It sounds to me like you've got some small tires at the rear. Tire size on the drive wheels also play a major role in the overall gearing.
With a TH350, 3.08 diff and 235/60/15 tires you'll get 25.3 mph for every 1000 rpm's meaning that at 3500 rpm you should be doing 25.3 * 3.5 = 88.55 mph.
Even with 195/50/15's you should still be getting 77.06 mph.
That therefore tells us one of three things:
1. Your speedo is inaccurate.
2. Your tacho is inaccurate.
3. You don't have a 3.08
It could even be a combination of the above as the 195/50/15's with 3.73 gears will still give 63.6 mph.