first off, find a new hobby store... your problem sounds like either you broke a drive cup... broke the front cvd(drive shaft) or the set screw in the center drive shaft is loose or fell off.. It sounds like you don't know alot about this truck... well, we will help you here.. check each area I mentioned and see what you find... I bet its one of those...
The dif cups are at the ends of each drive shaft in the front... if they break, which they will, you will lose the drive to that axle.. buy extras..... the drive shaft is less likely to break thankfully, as its expensive to replace.. the set screw is located on the driveshaft that runs from the two gears to the front diff of the truck... each end has a set screw and a drive cup... they are not too tight from factory and will get loose... check them and all other screws on the truck often... use loctite on these to prevent this from happening again...
Lastly... your hobby store selling you a lubricant spray shows lack of knowledge in the product or any product in general.. sounds like he just wanted to get rid of you....