When I hauled 2200lbs of roadbase in the back of my old '86 C10 (1/2ton), it swayed and wouldn't safely go over 35MPH.
When I towed my Corvette on a U-Haul trailer and had nearly 1000lbs of schtuff in the bed of my '82 C20 Diesel (3/4ton) moving back from Florida to Columbus in '08, I drove around 75mph for almost 15 hours with very little problem, even in stop-n-go traffic around Atlanta.
Up into the 90s, i think having a 1/2T or 3/4T really made a difference in towing, especially if you did it on a regular basis. HOWEVER, modern 1500-series 1/2ton trucks will tow 5000lbs just fine, especially if it's only a few times a year.
EDIT: Saw your post above GL with whatever you buy!