US Half tracks were regular US trucks, up-armored and the rear wheels replaced with a commercial track unit that replaced the rear wheels (On the other hand German half tracks were special made vehicles.
Exactly when the US replaced its M3 half-tracks is unknown to me, but none seem to have served in Vietnam, but the M59 and M75 Series of fully tracks vehicles were NOT produced till the Korean war and a good replacement for the Half tracks did not really appear till the M113 did in 1959 (Through it seems most US half tracks were gone by the mid 1960s, but even the Israelis had a concern about replacing their M3s with the M113 in the early 1970s with many Israeli officers questioning the superiority of the M113 given the M113s additional maintenance costs, costs in both time and money). The Israelis seem to kept some M3s still in reserves to this day, through that may be do to the fact the Israelis plan to retire those vehicles when the reservists tied in with those vehicles die of old age (and when a mean some I mean some, just a handful that do specialized services within the Israeli IDF and either on their way to a Museum or kept in one).
Torsion bars were first used by Porsche for the Mark 3 Panzer of WWII fame, all subsequent German tanks used them, almost all post WWII designed tanks used them (The Russian T-34, and American Sherman did NOT, but those are WWII Designed tanks with the T-34 converting to Torsion bar suspension by 1944). The only tanks suspension, other then torsion bar, used in post WWII designed Armored vehicles are the hydro-pneumatics systems first used in the XM-70 tanks of th elate 1960s, when the XM-70 was canceled do to costs the next US tank, the M1 reverted to torsion bars suspension, through the Japanese and Korean tanks adopted since the 1960s have used hydro-pneumatics suspension.
More on the XM-70:
http://en.wikipedia.org/wiki/MBT-70Torsion bar suspension:
http://en.wikipedia.org/wiki/Torsion_bar