8 3/4 geared to low. Whats compatable?

I've got a set of 3.91's in my 8.75 Tru Trac and it will shift through ALL 3 gears in a very short distance going at a slow pace. If you are getting 3 obvious shifts then yes you will want some taller gears for the road. The question i would have is the rear you have is it a sure grip? Keep that portion if it is. To change the ring and pinion is more of a chore that most novices dont want to tackle much less have the tools to accomplish the task. Swapping the whole chunk can be done in the drive as was stated earlier. When you pull the old one there will be a number stamped on the ring. Let us know what that number is. Just in case do remember, the ring bolts are LEFT hand.
Small Block