Ray,
Congrats on your purchase. You've got a great, capable vehicle that should give you years of pleasure.
That said, I'd be careful using it for heavy duty wheeling...especially mud. The Trooper is a "Light-Duty" SUV. It's not meant for mud-bogging and deep water fording. It's built for hauling lot's of stuff and trail riding. Your Trooper has a 10-bolt Isuzu corporate front axle and a 12-bolt rear end. That's as heavy duty as Isuzu offers, so you're in good shape there. The 2.6l 4 cylinder is a tough little mill. The valves need adjustment on a regular basis (every 15-20k) and that's pretty much it other than routine maintanance. I'd recommend upgrading your radiator from the single or dual core to a three core model, especially if you're going in the mud.
There are no other engine options for you without an entire drivetrain swap...and for that kind of money, you'd be better off buying a "real" mud bogger, like an old K-Blazer or Bronco. No other motor bolts up to the 2.6 tranny bellhousing (not even the 3.4 or 4.3 Chevy...they bolt to the 2.8l 6). The older 2.3l will, but that's a loss of 30 hp. I disagree that an entire drivetrain swap would be cheaper than a rebuild.
As far as lockers and limited slips, you can put an ARB air locker in the front (10-bolt) and maybee the 12 bolt, too...but that's about it.
Increased HP- Pacesetter makes a header. They work pretty well. You can also get a performance cam to help out on the bottom end.
Tires- 33's might be the max, but they should fit without cutting.
Lift- 3 options:
1)Suspension lift consisting of torsion bar crank in the front and heavy duty shackles in back. That's what I did and it was easy and gives you much better clearance than just a B/L
2)Body lift- 3" is available from link below
3)Both
I suggest you visit
www.independent4x.com for ideas and parts for Troopers and
www.4x4wire.com on the Isuzu forum. Great resources.
But if you want a serious mudder, you may want to sell the Troop and buy some good old American muscle...