The 2.6 has a belt driven fan. It is designed to increase the idle when the engine is hot. I have driven mine with the AC on in 100+ temps this summer and have never had the temp gauge go past verticle. If your's is doing that it definitely sounds like a cooling problem. Better get it fixed because the heads on that motor are prone to warping due to high temps. I wouldn't just remove the T-stat. That engine is designed to have one. In fact, if you replace it be sure to get a genuine Isuzu part. Some users have reported problems with some of the after market
Personally, I'd start with a good cleaning of the radiator. Sometimes mud and leaves get caught in the gap between the radiator and the AC cooler cutting off the air flow especially at idle. The next thing that I would try would be a good cooling system flush. After that if you still have overheading problems check to see that you are getting good flow. You can usually tell by feeling the top radiator hose, just be careful not to get burnt.
By the way, how is the timing belt? Do you know when the belt was last changed? If not, the belt can be inspected by removing a few bolts and bending back the plastic cover. If the belt looks worn, replace it. A busted belt will do a number on the valves. Most usually change the water pump (and belt tensioner) when they replace the timing belt.
Daily Driver: 1995 Trooper Limited, Auto, 3.2L DOHC, Rear LSD, HD Torsion Bars, Moog Cc812s, Cooper Discoverer A/T3 245/70-16
Trail Truck: 2001 Amigo, Auto, 3.2L, Rear LSD, Front Aussie Locker, HD Torsion Bars, OME 912s, BFG M/T KA2 33x10.5R15
RIP: 1988 Troop 2.6L 5spd. Burnt valve(s). Damaged frame. Parted out. Sent to crusher.
RIP: 1998 4WD Rodeo, Auto, 3.2L -- Ruined by Daughter #1 with Russian assistance. Parted out. Sent to crusher.
RIP: 1989 Troop 2.6L 5spd --- Stripped of all good parts. Body and rotted frame sent to the crusher.
RIP: 1999 2WD Amigo, Auto, 3.2L -- Engine blown and body damaged by Daughter #2. Parted out. Sent to crusher.