I can think of 2.9 reasons why it might be low. Probably about 4.56 ways to fix it.
That is to say, contrary to some member's beliefs that engine is not a powerhouse. I would assume that there were larget tires to go along with that lift, if so it will have an effect on performance. I know that a lot of people will change to larger 4.56 gears with larger tires to get the engine back into its power band, depending on what you have now even swapping in a stock 4.11 axle might be an improvement.
Has it gotten worse over time? An abrupt change from better performance? What has changed in that time period?
Low torque is not a symptom of a u-joint. Zero torque is, when it is broken. Clanking and banging is, when it is loose and on the way to becoming broken.
Even if you find a problem and fix it, I wouldn't count on much uphill acelleration with that combination. Plan ahead and acellerate prior to the hill, then try to maintain speeds moving up the hill.