The truck is a manual and is missing 4th gear...I guess you just can't shift into that gear.

Is it normal for the truck, when shifting to have to shift into 2nd by the time you hit 5 mph? Have to shift into 3rd, by about 25 mph. Then you have to shift into OD, by 35 MPH.

Also, What is the difference between 3rd/4th and OD? Just different ratios?

 

1-can you shift into 4th with the engine off?

2-this could very well be a normal condition, all depends on the first gear ratio.

3-yes 3rd gear ratio would provide torque multiplication, 4th would be a direct drive 1:1 and 5th would be an overdrive gear ratio.