Buying one new tire for AWD Astro Van 2002

I needed a new tire after a rear flat last week. I assumed I would be buying two tires for the rear. The tire dealer said because I have an AWD I have to replace all 4 tires so they are all the same size,(worn vs new). He said running new tires only on the rear would make the front tires turn faster than the rear because they are worn and are smaller in diameter. He said I could hurt my drive train or differential or something like that. I assumed he was just trying to sell me 2 extra tires. HOWEVER, while driving off the lot at a very slow speed I felt a kind of clunking in the steering wheel and back wheels. I have been feeling this all week, (only at speeds under

17mph). It feels like it starts in the front then transfers to the rear and then back to the front again, all in about 3-4 seconds. Was I wrong in my assumption, and should I puchase tires for the front as well? The new tires are the same size as the old ones. Joel

Your van uses the New Venture 136 transfercase which automatically locks the front and rear axles together when its computer commands it to do so.

What may be happening is that a significant difference in the new rear tire diameters are causing the rear axle to report a different speed to the VCM than the other 2 wheels. This is interpreted by the VCM as wheel slippage and the transfercase is instructed to operate its internal motor to squeeze a clutch pack together so that all of the wheels get traction.

This motor is a stepper design which can be operated very fast and since the perceived slippage technically is never corrected, it may be squeezing in and out constantly, causing a driveline pulsation.

It is possible that the sensation goes away above 17 mph because the VCM stops correcting for slippage at that point or that higher speeds of the driveline components absorb the pulsations better.

Your tire dealer was being honest, but its always safe to get a second opinion.