OK, my 2 cents. (OK, after reading what I wrote, it looks like closer to 3 or 4 cents...)
MORE POWER:
AUX PUMP:
When the thing is groaning to move in the higher ranges, where is the accellerator pedal? Mashed to the floor or just barely off idle or somewhere inbetween?
The transmission has 2 pumps. Normally, only the primary is in use. It offers max speed and medium power within the particular gear (H, M, L).
For more power (but at the cost of speed), you'll want to kick in the secondary (aka. Aux.) pump for more power.
Finding the "sweet spot" to engage this aux pump takes a little practice but you'll find it at about 1/2 pedal. The best way to find it, get it to kick in and get used to it is hill climbing (if possible). If no hills, you'll need to put the tranny under load by some other fashion.
Here's the easiest hill test. Put the RTV in L range. Get it up to full speed (keep the pedal mashed to the floor) on approach to the hill and let it start climbing the hill. Once the engine starts bogging down, gradually let OFF the pedal to near the 1/2 way point. You'll then notice your RPM's and power going way back up. You'll then have tons of torque but you'll be traveling slower since the aux pump is engaged. If the RTV doesn't bog down because the hill isn't steep enough, do the process in M, then even in H.
TRANNY FLUID
It's been found that many cases of lack of power are due to clogged tranny filters. A regular change of the tranny fluid and filters will/should assure this isn't the issue. Don't use fluids other than Kubota's UDT or SuperUDT as some have shown to degrade performance significantly just by using that fluid.
2 VS. 4WD
You'll normally get a lot more torque in 2WD. 4WD just has a hard time turning everything...
MUFFLER
I don't have my owners manual handy but there's a couple steps to cleaning.
1. Remove the spark arrestor off the end of the muffler pipe and assure it's clean (not clogging up).
2. Remove the drain from the bottom of the muffler and run the engine at about 1/2 RPM's for a few minutes. There's more details but I don't recall what they are...
LIFT KIT
Unless you need it due to a significant change in tire size, I wouldn't recommend it. I put new tires on mine and ordered 25" tires (same as OEM). Well, the 25" Maxxis tires stand 26.5" tall. I was getting a slight rub on very extreme conditions so I installed the lift kit. With the oversized tires, coupled with the lift kit, the center-of-gravity is about 3-4" higher. It is noticable and feels a little more tippy.
OIL CHANGE
Requires 3.1 - 3.3 quarts. NOT 2.1 as noted in some of the manuals.