That is the unfortunate problem with the 545RFE - the poor spacing between gears, they are simply too far apart. Like you described, the transmission downshifts and you have power to accelerate, but once it shifts up a gear it is simply too far and the engine begins to bog down and lug until it has to downshift again. That is why I have found it is better once it downshifts to manually select the lower gear to hold it (whether it be 2nd or 1st) until you top out on the climb or the grade lessens. Once in the lower gear, if the RPMs climb to 4800, I will back off on the throttle to keep from going over 4800 and the speed will be whatever it is. I watch my tach on the pulls more than the speedo, I'm not going to be speeding up a hill with the 5'er anyway! :)
I am a little confused about your RPMs. Is your 4800 RPMs @ 50 MPH in second gear accurate or are you just making a statement? I ask this because with 3.73s and larger than stock tires, your RPMs at a given speed in a particular gear should be lower than mine with 4.56 gearing. In 2nd gear @ 55 MPH my RPMS are 4500:
I can usually hold this on 6% grades up until around 5000' to 6000' elevation. Above that my speed will begin to decrease as I go up in elevation.
In your case it sounds like regearing would give the best results. The cheaper option is to be patient and settle for a lower gear and speed. I have yet to find anything I can't pull my 5'er in first gear. That includes 10% grades and elevations 9000' and above. I have towed our 5'er on I-70 to the Eisenhower Tunnel at over 11,000' three times.
Good Luck!
