umm. im thinkin rebuild on the tranny no? but i could be wrong. since it only does it in 2 gears it makes me point towards the tranny, not the flywheel or the clutch. the first thing i would of said would be drain and fill w/ some mt90 but you already did that so lets see what some experts have to say about this.

87 22R PU, 2? body lift, 33? BFG muds, 4.88's, F&R Lock-Rights, Rancho Shocks, EB's Street RV Head w/ 268 Cam, LCE Header, 2" to 2 1/4" exhaust, Flowmaster 50 series, Magnaflow high flow cat, complete Herculined SR5 interior, V6 BB, MC, and calipers.