I've driven the KR21s tens of thousands of miles, but on cars not trucks. The XL sidewall construction and shallow tread depth should make them less "squishy", but???
KR21: more of a car tire, 11/32 tread, XL sidewall for 2183# rating, 50 psi, 85K mi warranty, weighs 28#, Part #2104463 (they make a standard load with diff # -- you don't want those)
KR21:
KL51: more of a crossover tire, 14/32 tread, C load for 1985# rating, 50 psi, 60K mi warranty, weighs 34#, Part #2103023 (they make a standard load with diff # -- you don't want those)
KL51:
KL61: more on/off road truck tire, 15/32 tread, C load for 1985# rating, 50psi, 60K mi warranty, weighs 35#, Part #2114533 (they make a standard load with diff # -- you don't want those)
KL61:
Edit: I'd get the KL61 before the KL51.... But the higher load rating and long life of the KL21 might be the best choice