A local speed shop recommended that I run a tapered piece from the muffler out, after I lost the low end torque with the headers/hi-flow cat and muffler. I figured I would try a different muffler first, before trying the smaller tailpipe. I went with a raptor turbo from Kragen's, and gained the low end torque that I wanted. I cruise at about the same rpm's you do, rarely get over 4k. With the set-up I have I cruise at about 3k at 70 in 5th(33's and 4.88's), and I have enough lowend torque that I can almost idle along in Hi-2wd.

I would recommend you run the 2.5 pipe and see what it gives you with headers/cat/muffler you havenow. If you want more torque try a different muffler, or a diff tailpipe. If you run the 2.25 pipe first and it is too restrictive, you can't do anything to make it flow better except to replace it.