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No such thing as a u boot, unless you're talking about a WWII German submarine. You probably mean u-joint. Do a search and you'll find a ton of posts explaining how to replace them.
U-joint is shorthand for universal joint. If you get under the truck you should see the driveshaft running from the transmission to the rear axle. There is a universal joint at the transmission end and another one at the axle end. With transmission in neutral and the parking brake on try shaking the driveshaft at each end, you should find which joint has play in it.
I wouldn`t have thought a replacement joint was a junk yard part.
Hope this helps.
U-joints are a wear item. You can't (ok, well, you shouldn't) use a junkyard for wear items. You wouldn't use junkyard brake parts, right? I hope? I order mine online. They are cheaper and much better quality than the stuff the local parts stores usually sell.
Buying u-joints from a junkyard is a terrible idea. Esspecialy for $20. You can go to Schucks and get their cheapest u-joints for around $11. Like Lazy K said, get under the truck and find out where the play is coming from. Then drop out the driveshaft and replace the bad u-joint, or more preferably replace both u-joints on that driveshaft. I always replace all of the u-joints on a driveshaft if I have the money. When one goes, the others ussually are not far behind.
To replace, remove the clips with needle nose pliers and a screwdriver (to pry with), then use a socket similar in size to the bearing cups and a hammer to punch them out. Then gently punch the new ones in and replace the clips.
I don't currently have a large enough vice, thats why I mentioned the hammer method. If you have a vice, that is the easiest way to do it without damaging anything. Autoparts stores that rent tools usually have a ball-joint press that will work on u-joints, Harbor Freight also sells this tool for around $35. I have used this type of tool to do ball joints and they work great.