depending on the tire size and gear ratios, the t150 does rather well at 60-65 mph. i was cruising today at highway speed and it wasn't straining a bit with my 31's.
i'd say stick with the 3 speed for a few months and see how it works out for you. its a very strong gear box and i personally really enjoy the feel of it. its like driving a big truck, long shift throws and you can feel each gear engaging when you shift. nothing like driving a TJ IMHO.
'06 Wrangler SE, soft top, hard doors, 2.4L, rolling completely stock and unmodified for the short term...
'10 Wrangler Sport, soft top, hard doors, 23S package, Teraflex 2.5" spring lift, Spidertrax 1.5" wheel spacers. Sold.
'79 CJ-7 Renegade, 258 I6, T150, D20, D30/AMC20, 2.5in Rancho lift, 31x10.5 AT's, TeamRush upgrade, flush tail lights - Sold :'(