1987 gmc sierra

hello all, im new to page. got a new truck its a 87 gmc sierra, i love it its a good work truck, but   its has the 4 speed standard with the granny gear and then 3 other gears. i was wondering if it would be a good idea to put a different tranny cause when im in third (final gear) goin down the road at like 45 or 50 it seems like the motor is revved up to like 4,000 rpms, and sucking the gas down some one put aa 1 ton rearend in it also.   i just use it for work but if i can get it to save gas and not wear out the motor i would like that.. any suggestions?  

Something is wrong.  By my calculations, even if you have 4.11 gears, it should only be turning 2400rpm@50mph, with 235/75R15 tires.
If you go with different gears it drops as follows, considering 235/75R15 tires:
2.73 : 1600@50mph
3.08 : 1800@50mph
3.42 : 2000@50mph
3.73 : 2200@50mph

If you are doing something silly, like little tiny tires, then you could well be approaching the 3000rpm range.  
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1949 Chevy 1 1/2ton, home-built hydraulic wrecker.  Rustoleum industrial black, with flames.  Flamethrowers out 5" dual stacks.
1962 Chevy C-20, 402BB, 4-speed, 3.90 positrack. Hauls my camper.... among other things.

well i do not have a tach in the truck but its running the stock 16 inch wheels   its the 3/4 ton  it definintly is wanting another gear once you hit 50 mph  i let off the gas and it boggs down and slows the truck down like a engine brake i dont wana put a bunch of money in it but i wana good driver its deffinitly a hauling truck it would pull a house down.   thanks for your reply.