14k on a 3/4t suspension? whew..
3.55s with 34" tires is going the wrong direction, if you're looking for pulling power. You "virtually" replaced your 3.55s with like, a 3.23, as your speedo (uncorrected) is about 5-6mph off (reading slow). This should be a solid 3-400rpm lower than OE size tires.
I want to replace my 3.55s with 4.10s, but I rarely do more than 65mph, and I tow pretty much constantly during the spring-to-fall months, at ~9000 lbs.
Additionally:
3.55:1 axle
1800 rpm
34" tire diameter
1:1 tcase ratio
.69 final gear (auto OD)
YIELD: 75mph
3.55:1 axle
1800 rpm
31" tire diameter
1:1 tcase ratio
.69 final gear (auto OD)
YIELD: 68mph
A 4:10 diff gear set will drop that 75mph @ 1800 to 64mph @ 1800. Respectively, the 31" tire at 1800rpm would drop the speed from 68 to 59mph.
The truck would pull a lot better, imo. The top end would be a lot lower, but I think the more responsiveness and applied torque would outweigh that. At least, it would for me.
Remember, that the 3.55 and 4.10 in a D80 axle are different carriers. I think the 3.55 carrier will take up to a 3.73, then above that, requires a different carrier assembly.
---edit---
My truck has the D80 LSD rear. If I were to go with a locker, my first thought it to look at Powertrax. This is because that's what I used in all my rock crawlers, and they performed very well. Albeit noisy, I really liked them. Take into consideration, also, that a locked rear will be more skiddish and a handful to drive in 2WD mode on rainy, snowy, and generally slick conditions on hard surfaces..