I have an F150 4x4 with a 5.8 and 3.55 rearend. What would be the largest 5th wheel I could pull without having to get out and push to help the truck. Most of the camping areas are in the Mtns. at elevations of about 4 - 6 k feet.

 

Check your owner's manual! Mine has a chart showing various truck & powertrain configs.
I have a Super Duty w/V10 and my 5th wheel limit is 13,300 pounds with a hitch weight of 1931 pounds.
I've only seen a few 5th wheels that would be light enough for you. Also, watch the hitch height! With a 4x4 the hitch is higher and it could make you drag your trailer's bumper. You may need to raise your trailer by re-mounting the axles below the leaf springs. That'll give an extra 3-4 inches. Also check with Ford. I understand they will LOWER the rear of a 4x4 for trailer towing FREE. (At least they do for Super Duty's)
Good Luck

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1999 F-250 XLT Super Duty Crew Cab Long Bed
V-10, Automatic 4R100, 4.30 Limited Slip.
GVW = 6700# Towing Capacity = 13,300#
4 Wheel ABS disks + Reese 15K hitch
Dark Hunter Green. Ford Running Boards.
Grey Rhino-Liner. Captain's Chairs. CD+Tape
Polished Stainless Steel Wheel-well Molding http://members.home.net/jchartier/jnctrk.html