I run 4.88's with my 35's. Thats on the "built" but stock axles. If I could go lower I would, 5.13, but the stock axles are limited and actually the lower the ratio you go on the D-30 the weaker the ring and pinion is.
I will say that 4.88 is perfect on the highway with the 35's. I cruise along getting good mileage with that ratio.
I like lower gears, my DD 90XJ has 4.10's and stock tires. I live in the Blue Ridge Mountains(think short but steep) and those billy goat gears are a nice addition to any XJ around here.
This is my trail rig, on the highway, speedo is dead on right checked with my GPS. This thing cruises very nice!