It really depends on how much articulation you have on the trails. If there isnt much you can get away with 4-5 inches. They with rub a little, but will clearence themsevles out and doesnt look bad. If you have a lot of articulation I would recomend at least 6 inch lift. I run 35x12.50s on a 4 inch lift and articulate alot. It sounded bad the first time they rubbed, but after two times out they quit rubbing, and you couldnt tell they pushed them fender back. I went slow though because I knew they would rub. If you go fast it could cause alot more damage.
86 Jimmy 4"lift 35"MTRs locked front.Other cars 69 Firebird w/525 hp,and 70 GTO <a target="_blank" href=http://www.geocities.com/modifierperformance>MY vehicles</a>