Ok, just to clarify:
"All you need to do is suck out the R-12 and oil, replace the oil, and fittings, and add R-134a".
NOTHING is further from the truth.
These trucks do not convert all that well, from personal experience.
To *properly* convert it (for ideal performance), you need to replace the fittings, suck out the R-12 and drain all the oil from the compressor. All the old oil needs to be removed, because it is not compatible with the PAG oil in the R-134a systems. When they mix, they become acidic, and will slowly destroy the A/C system.
You need to replace all hoses. R-134a vehicles have "barrier hoses" which prevent the R-134a from escaping, as the R-134a molecules are much smaller than the R-12 ones.
You need to replace the orifice tube with one designed for R-134a. R-134a runs at higher pressures than R-12, and R-134a parts are required for optimum performance.
You need to replace the Accumulator/Drier (silver tube by the firewall), as whenever the system is opened, it should be changed.
You should replace all the O-rings as well, and verify the A/C compressor isn't leaking from around the compressor seals, clutch, or case.
Once all this is done, oil the compressor, and put the system under a vacuum for at least 30 minutes. If the pressure doesn't drop, add R-134a. You only need 80% of the R-12 charge.
From experience:
R-134a simply does not cool as well as R-12. With R-134a, expect 40-45 degree vent temps, providing the system is working as efficiently as possible. If it isn't, expect 50-55 degree air. With R-12, we're talking vent temps in the upper 20's, lower 30's. It also takes it a lot less time to drop from 90* to 30*. It is more efficient than R-134a, thus requiring a smaller condenser. When R-134a is added, the system has to work harder due to the higher pressures, and has less than optimal airflow from a condenser that is undersized, preventing the temps from getting as cold as they could be, and as quickly.
Your best bet, since the truck is simply low, is to buy a few cans off eBay. Look for sellers that will accept a letter saying (basically "I, William D. do hereby certify that I am purchasing this freon to re-sell to a certified technician for installation."
In all honesty, the sellers don't give a sh*t who buys the R-12, as long as they can make a buck. eBay forces them to have the "must be EPA certified or resell to certified tech" in their listing, to eliminate any liability.
I should add, for those still considering switching to R-134a, I'm currently running R-12, and even with literally 50% of my condenser fins bent shut (working on getting a comb to fix that), I'm getting 40-44* air. Sometimes it's even in the upper 30's. I'd imagine once I double the airflow through the condenser, I'll get another 8-10* air drop. My A/C is so cold I have to turn it off sometimes on 90* days just to thaw myself out a bit.
here. no shops will touch it, is there any options for me or am I stuck replacing everything. or is it really just as simple as changing the fittings, flushing it, drawing a vacuum to ensure it holds, then refill with oil and r134a? or is there more to it... with my wife starting back to work I occasionally have to pick up the kids in the afternoon and this heat is too much for the little guys. any help will be very appreciated and welcome! also if there is a better place for this thread feel free to move it 
