My '88 is also of the carbed variety. They started putting EFI in them in '90 IIRC. The 1.3 L engine is by far the worst part of the vehicle. Mine has bad valve seals and fouls plugs every 13,000 miles in addition to going through a quart of oil at every other fuel-up. Top speed with 2" lift and 29" tall tires is 62 mph on flat land with no wind. I'm told that stock examples with fresh, stock engines will go 75.
Engine swaps such as the 1.6 L 16V engine out of a Tracker or Sidekick is said to be pretty easy and yield about a 50% power increase. Another option is the 1.6 VW diesel. I was actually planning the latter upon the impending failure of the current motor, until I decided to move and sell the Sammy later this year. People have done the 4.3 V6, but this is really overkill for a truck that weighs a little over 2,000 lbs.
With the tired original engine and 193,000 miles, I get between 22 and 25 mpg depending on how I drive and whether I leave the hubs locked. I use it for my daily driver on a commute of 30 miles paved and 20 miles "not paved" (varies from relatively smooth gravel with some washboard in the summer to mud, potholes, ruts, standing water, etc in winter. After a year I have had ZERO suspension problems, which is much more than I can say for the Lincoln Town Car I used to use for that duty. I keep the tires at 21 psi, and the ride quality is not great but certainly acceptable.
http://www.pirate4x4.com has a discussion area with some rabid Sammy lovers.