GM Repair Questions? Ask a Mechanic for Answers ASAP
First of all what is the truck you have now, a K1500, K2500 or K3500? Chilton says the front diff on the K1500 and K2500 is a GM 8.5" 10 bolt. The front and rear ring and pinion ratios should be the same. The rear diff you say is a 10 bolt so it is most likely the 8.5" as well.
What is the parts truck diff's? I know you said 14 bolt, but are they 9.5", 10.5"?
Here is a chart,
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If right now with those 36" tires and at 60 mph you are reving at 2,050 rpm with the 3.73 gears perhaps switching to like a 4.56 set will raise the rpm to about 2,500 rpm.
If the 14 bolt is a 9.5" you can get a set of 4.56 gears here http://www.drivetrain.com/parts_catalog/ring_and_pinions/gm_9.5_ring_and_pinion_sets.html#9.5
If its a 10.5" here are the 4.56 http://www.drivetrain.com/parts_catalog/ring_and_pinions/gm_10.5_ring_and_pinions_14_bolt.html
Now you are going to have to put 4.56 on the front.
Here is the 4.56 for the front 8.5"