While the 85 is the "choice" year if you want to leave it alone, if you plan on doing any major mods for trail use I'd convert a 86-89 from IFS to SAS because you end up with stronger spring mounts than factory, a wider rear axle with thicker tubes, and bigger rear brakes, along with an IFS steering box already on the frame for Hy-Steer. The 22RE is the most reliable powerplant and accepts upgrades easily, already has the better W56 transmission & the best 21 spline gear drive t-case. ALL of the 4 & 6 cyl automatics had chain drive t-cases, but can now be converted to gear drive cases with adapters from inchworm or gearslug. The V6 manual trans is a good R150 23 spline setup, but still has the chain drive case which can also be replaced with a gear drive case when you get an adapter from AA for about $375. If you plan on doing a drivetrain swap (like a 4.3 V6 GM & TH700R4 auto) then find yourself a V6 with a blown headgasket for cheap & convert it, as you will have the stronger V6 rear diff with its larger carrier bearings and extra webbing on the pinion area. This V6 diff can also be easily swapped in to the 4 cyl housing if desired as a strength upgrade.
Terry
APOR