The amount of weight the tongue will add to the rear axle can be simply calculated:
R = rear axle weight added
H = distance from ball to axle
W = vehicle wheelbase
T = tongue weight
R = T ( 1 + H / W )
If you have a typical long bed crew cab, even with a 36" extension, it will only be about 1.5 times the tongue weight.
R = 250 * ( 1 + (36 + 36) / 168 ) = 250 * (1 + 72 / 168 ) = 250 * 1.4 = 350
Measurements estimated to show an example.
For those light of trailers, a 2-3' extension wouldn't be a concern at all.
Just make sure the tongue has a very good coupler since the ball will be thrown up significantly going over bumps with it being that far behind the rear axle.