AC runs cold for initial driving but if we stop or slow down then as we move again goes warm...but as speed increases blows cold. Cooling then dependent on speed. Slow-warm, faster-colder.

Thoughts

 

My crystal ball ain't what it used to be and I am having trouble figuring out what you might be driving, And, since all vehicles are not the same, I don't know if the electric cooling fan is working if there is one , or if your vehicle has a fan blade mounted on the water pump etc., But, anyway all kidding aside , those 2 items might be a good idea to check, whichever one applies.

 

What year truck?

When it blows warm, lift the hood and see if the AC compressor clutch is engaged and spinning the comp, Yes/No?
You can also grab the AC line at the evaporator case at the firewall, it should be very cold or even some condensed icing present if the system is heathy and active.

 

Also, depending on the year of your buggy, you may be a little low on refrigerant causing the compressor to short cycle at low speed/idle. This would also cause your scenario.