I wasn't going to mention the NV4500 as an option...
As LB55 said... getting a freeway friendly differential will reduce available torque and acceleration in every gear.
Assuming you have a sm420 in your truck now, first gear is not used much. However when it is used there is a reduction of about 27.5:1... meaning your engine will spin almost 28 revolutions before your tires will spin 1 revolution. This reduction maxismizes and multiplies the torque.
The sm420 has about a 7.05:1 first gear, and no overdrive. An NV4500 has a pretty low first gear (5.61:1) and a 0.73:1 od. The gear spacing of the nv4500 from 1-4 is almost identicle to the sm420 in 1-4. The big payoff is the OD gear. There is a slight loss of torque in 1st gear, but a big drop in rpm in 5th.
So to answer your quetion. Changing the differential will affect acceleration and applied torque in every gear. This could make your 235 feel sluggish. Swapping the granny gear for the OD gear will give you more speed, but less takeoff power at the boat ramp. Using a NV4500 (likely overkill on a 235 powered 1/2 ton) will give you a bullet proof transmission that can pull a boat out of the water and down the road reasonably well.
Every modification you make to the truck over the original confiuration is a trade off. If you use an OD transmission, you lose the granny gear. If you add add an OD behind the transmission, you can keep your granny gear, but you will lose your lunch money for a year. The NV4500 swap is a rather expensive swap. The t5 swap is a fairly inexpensive swap.
As far as Stand alone Overdrives, there are a few available. One is a "Ranger Overdrive". This unit goes between the bellhousing and the transmission. It will push your existing shifter lever back about 6.5 inches, and an an extra shifter for the ranger.
Another Stand alone OD is a Gear venders OD. These typically cost about $2700 new, and go between the transmission and the rear axle. This unit as well as the ranger unit allow you to gear split giving you about 8 possible forward gear ratios.
