I have a 1995 ford ranger 4.0 automatic and sometimes when I put it in drive it doesn’t engage right away and it even does that when I put it in reverse I usually tap the gas a little bit and it engages most of the time.

 

Automatics run on ATF fluid pressure
The clutches and Bands(brakes) are all controlled by fluid pressure
The Valve Body routes that pressure to the clutches and bands to engage and disengage the clutches and bands

This pressure comes from the Front Pump(only pump), it is connected directly to the torque converters outer housing so spins at engine RPMs
This fluid pressure is then routed thru the Valve Body which directs it to the clutches and bands for the gear you have selected
Forward gears require 125psi pressure
Reverse requires 175psi, <<< this required higher pressure is why automatics often lose Reverse FIRST when there is a problem


OK, so from your description it reads like you have lower pressure, raising engine RPM increases Pump speed so increases pressure enough to engage gears
There are gaskets and seals that can leak pressure, additives like Trans Fix can swell gaskets and seals so they leak less, these are safe to use and not too expensive to try

In the old days the Valve Body used ball/spring valves and vacuum modulators for "automatic" gear selections while driving
In 1995 Rangers got the 4R55E, it uses 6 electric solenoids instead of the ball/springs and modulators to operate the valves
There is one solenoid called EPC(electronic pressure control) solenoid
It can cause a problem, but the earlier 4R55E also have a problem with Separator Plate gaskets leaking pressure, Ford changed it to a Bonded gasket in later years

You can change the Valve Body without pulling out the transmission, it can also be updated with a shift kit and bonded separator plate