Nope not a serious matter at all! NO YOU DON'T HAVE TO GO GET ANOTHER TRANSMISSION, Last thing you want to do is go buy a transmission at the junk yard, you are just going to buy another ticking time bomb!
All you need to do is replace the 3-4 shift solenoid and I would recommend that you replace the 3-4 servo piston spring, even if it doesn't need replacing.
Here are a couple links for the parts, oh yea you are going to need a gasket kit for the oil pan. So go to advanced auto and buy a gasket and filter kit for the 46RH (covers the 46-48RE/RH in your year). Cost is like 5 bucks...
You will need a torque wrench, an inch lb torque wrench. Be SURE YOU DON'T OVER TIGHTEN ANY OF THE VALVE BODY BOLTS! Otherwise you will wish you hadn't of even touched your truck.
You are going to have to remove the valve body in order to replace the solenoid, if you search through all my posts I have a manual in there somewhere. If you cannot find it just tell me and I will give you the run down, this is if another member doesn't answer your question first.