I can't speak of current archaeological work at the site of Jam, but I do know a fair bit about the Ghurid dynasty.

The dynasty that is famous today was likely one of a number of Iranian clans that vied for power over the region of Ghur. C. E. Bosworth calls them Tajiks, but Tajik generally refers to Persian-speaking Eastern Iranians, and I think it unlikely that this fairly isolated group in the Hindu Kush were Persian-speakers. More likely, Bosworth just means that they were Eastern Iranians of some sort.

The tenth century Persian geography Hudud al-Alam claims that Ghur was under the suzerainty of the Farighunid dynasty of Guzgan, to the north, and that the people of Ghur were Muslim. However, the anonymous author was writing at the court of Guzgan, as he names the Farighunid king as his patron. Furthermore, the author's knowledge of Ghur is surprisingly limited given how close he lived to the area.

The religion of the Ghurids was likely some form of Indo-Iranian polytheism, related to Zoroastrianism and Hinduism. Some modern authors have claimed that they were Buddhist, which is entirely plausible, but unfortunately I haven't been able to find evidence of this. From my own research, I would argue that they were converted to Islam by a combination of the Chishti Sufi order and the Karramiyya theological school. As you may be able to work out, Chisht is a town just to the west of Ghur. The Sufi order there was founded in the tenth century by a native disciple of a Syrian Sufi. The Karramiyya school on the other hand, had a few interesting features: it advocated the idea that declaring yourself a Muslim was enough to make you a Muslim; it encouraged asceticism; and it encouraged preaching an easily understandable form of Islam for the masses. Of note, the early Ghaznavid dynasty were also supporters of the Karramiyya, so it may have been from that direction that Islamisation in Ghur occurred.

During the eleventh century, the clan which became the famous Ghurid dynasty managed to achieve supremacy over the other clans, possibly with the help of the Ghaznavid dynasty. To clarify, the Ghaznavids were of Turkish mamluk (military slave) origin who began as governors for the Iranian Samanid dynasty. The Samanids were also the overlords of the afore-mentioned Farighunid dynasty. As the name indicates, the Ghaznavids were based in Ghazni, to the east of Ghur. Throughout the eleventh and twelfth centuries, the Ghurids appeared to have traded allegiance between the Ghaznavids and the Turkish Seljuk dynasty to the west. During this time, one of the Ghurid rulers was executed by the Ghaznavids, which looks to have led to some lasting enmity between the two dynasties. The collapse of the Seljuks in the mid-twelfth century likely gave the Ghurids the room they needed to expand, mostly at the expense of the Ghaznavids.

By the end of the twelfth century, the Ghurids had managed to eject the Ghaznavids from their base in Ghazni. The Ghurid dynasty at this time was jointly ruled by two brothers named Mohammad: Ghiyath al-Din and Muizz al-Din. One was responsible for rule in Eastern Iran, while the other was responsible for the expansion into India, where the Ghaznavids had previously conquered territory. Earlier in the dynasty's history their armies were most likely dominated by the tribal Ghuris. However, by the end of the period, the dynasty had become reliant on Turkish mamluks. This army conquered much of northern India, including Bengal. Following the death of the two brothers, the subsequent infighting between the various factions (most of whom involved the mamluks) led to the disintegration of the empire. Perhaps the most important of this sequence of events was the creation of the Delhi Sultanate. Indeed, as brief as the Ghurid empire was at its height, they were arguably one of the main forces behind the Islamic conquest(s) of India.