Probably the best-known Buddhist monastic archeological site in what is now Tajikistan is at Ajina Tepa, which has been diligently excavated and published by Litvinsky and others. It’s a fascinating site.
Lesser known, and less impressive in size and sophistication if not in location than Ajina Tepa, is the stupa at Vrang, on the Afghan border in the Pamirs. (It’s at 37° 0′ N 72° 22′ E.)